Any person, given suitable conditions, has 2.5 hours with which to cultivate a state of jhana. After 2.5 hours the requisite causes for jhana have, accordingly, "passed by" and if such causes for the production of a jhanic state are not taken advantage of during those times, to the best interests of the person involved, then his opportunity to support such said jhana will also have been lost.
The reason is that the body is divided between various "functions" throughout the day. For example if a man was to wake up at six in the morning we might expect that his first actions, once getting out of bed, would be to -more than likely, empty his bladder. Following the call of nature a man will find himself, at around 8:30 in need of a movement ala las bowels. By the time the intestines have sufficiently done their work, it will probably be near 11:00 in the day.
At around 11:00 the a man will probably want to eat, and so his stomach is of divested interest to him. For the remaining two hours the stomach will busily invest itself in the digestive process (assuming he eats something).
Around the time that 1:30 comes around, the food eaten earlier in the day is by this time fully digested, more or less. The body then turns its attention to the heart, which at this time finds itself direly hungry for a quickening of sustenance. So it becomes the body's focus for the next two and a half hours. And, obviously, during this time the blood carries the nutrients absorbed through the stomach to whatever parts of the body happen to be in need.
This is where the logic of my understanding begins to waver on account of deficiencies in my knowledge of bio-chemistry. As such, I cannot strictly account for why, at around 4:00, the body focuses its attention on the thyroid gland, but a quick glance at a Wikipedia entry for "thyroid" explains as much as I need to keep going along this vein.
Following the dispersal of nutrient rich blood from the heart, which has been the main focus of the body for about 2.5 hours now -from about 4 to 6:30, the enzymatic activity controlled by the thyroid gland
becomes the focus of the body. The gland appropriately releases its contents which regulate and control how the body will continue to process the nutrients (now fully exchanged throughout the body by the heart), following the complete digestion of food by the body (which occurred naturally after the expulsion of bodily waste; specifically in the order of stomach waste following urination).
As you might expect around this time (6:30 to 9:00) the body turns its focus upwards once again towards the next relevant structure of the body in this climb towards the top. The pituitary gland
is responsible for bodily homeostasis, and as such -with the nutrients of the body having been regulated by the thyroid in the previous 2.5 hours, hormones from the pituitary set into motion a dance of biological reactions which further (de facto) set the appropriate needs of the body to a level which is neither too high, nor too low.
Finally, having accomplished nothing more than all one might expect the body to do towards the goal of living another day, it's 9:00, and the body will initiate all forms of repairs in so much as it is capable of doing this. When 11:30 rolls by most working people are ready to fall asleep and they do, or already have.
So obviously I am referring to functions of the body in a way that befits the theory of chakras, without any need to actually bring this controversial topic into discussion. Why did I waste my time writing out all of these things? Well, to prove a point. I want to say something about jhana.
There's little to be known about jhana from buddhist texts. Here's an example which practically encompasses all that is mentioned regarding such states. From http://www.accesstoinsight.org/ptf/dham ... jhana.html
"There is the case where a monk — quite withdrawn from sensuality, withdrawn from unskillful qualities — enters and remains in the first jhana: rapture and pleasure born from withdrawal, accompanied by directed thought and evaluation. He permeates and pervades, suffuses and fills this very body with the rapture and pleasure born from withdrawal. There is nothing of his entire body unpervaded by rapture and pleasure born from withdrawal.
"Just as if a skilled bathman or bathman's apprentice would pour bath powder into a brass basin and knead it together, sprinkling it again and again with water, so that his ball of bath powder — saturated, moisture-laden, permeated within and without — would nevertheless not drip; even so, the monk permeates, suffuses and fills this very body with the rapture and pleasure born of withdrawal. There is nothing of his entire body unpervaded by rapture and pleasure born from withdrawal..."
Notice in the simile that the first jhana is compared to a brass basin fully saturated with moisture laden powder. Notice that the skilled bathman or bathman's apprentice pours the bath powder into the basin and kneads it, sprinkling it from time to time until the whole of the water is saturated into one fine soap ball throughout the entire basin.
Now, what exactly are these terms supposed to relate back to? Well the bathman is, as I understand it, the conscious will of the being. The basin is the body of the conscious being. The fully saturated soapy water is the full entrance into jhana. The only question is; what should we compare the bath powder
and the kneading
of it to?
Well, as I mentioned, the body is busy keeping itself alive throughout the day in periods consisting of 2.5 hours. The start of the day involves getting rid of the previous days waste. The middle involves consuming and digesting sustenance. The latter half involves the biochemical transmutation, deliverance, regulation, and optimization of afore mentioned sustenance. Hence, if we are to really attempt an answer to the question of jhana that satisfies the elements of the simile above we must account for the question of the bath powder, and the answer comes in the form of what you might call "hormones" or "enzymes" of a physio-psycho-teleological nature.
Obviously a skilled bath man does not dump all of his bath powder into a basin at one time. He fills the basin with a generous amount, kneads it together and sprinkles in more, so as to fully emerge the basin with soapy bubbles. Analogously, if for example the bath powder mentioned here was, say, a sort of "substance" that eluded all forms of description apart from one might only recognized in terms like metta, mudita, karuna, or upekkha then we can at least account for how, in meditation, one slowly but surely arrives in a state of fully pervasive jhanic rapture.
When the time is right the physio-psycho-teleological bodily elements naturally come into their own right and will appropriately do so for the next 2 and a half hours. But, if a person wisely accounts for these "substances" and (like the bathman) pours a generous amount of them into his body, wisely opening his up "channels" for the acceptance and activity of the substance; without making the mistake of divesting all the quantity of the elemental substance in one full go, -but rather approaches a heightened state of mind through a gradual and intentional, mindful "letting go" and "churning" of these substances, he will, according to the nature of the physio-psycho-teleological substance, enter into each respective state. Because he has two hours to patiently do this, he may, if he is skilled in jhana, find himself immersed in the same state for a long time before, so to speak, the bubbles gradually pop and the jhanic state comes to an end. 2 and half hours regulate the time one has to invoke the jhana, but it does not necessarily limit how long one may remain in such said state.
So, who can say how long each state lasts for? If you have at the most 2.5 hours to cultivate the state, and perhaps if your ability to cultivate the state depends also on the amount of the emotive substance within you, the state could last for quite along time when you consider that some people are simply endowed with greater amounts of empathy, compassion, equanimity and so on and so forth. It all depends on how you sprinkle the substance and how you patiently wait for the object to come about -with the proper channeling/kneading that is required for the element to pervasively fill your body. It also depends on whether or not you know where these substances lie inside the body.
Immaterial jhanas are no different.