There are many possibilities:
- Someone ordains with the intention to remain for life, but his intention changes after a short time or a long time, and so he disrobes. Then he changes his mind and ordains again. There is no limit to the number of times someone can be ordained. As long as he did not commit an offence of defeat, and is still eligible for ordination due to being free from debt, etc., he can ordain again.
- Someone ordains with the intention to disrobe after a fixed time limit, whether one day, one week, or one year. While a monk, his intention changes, and he decides to stay a little longer. Time goes by, and he eventually stays for life.
- Someone ordains without any notion of how long he will remain, but just on a trial basis: as long as he is content with the monk's life he remains.
Some say that temporary ordination is worthless, and so discourage a potential candidate by saying, “If you cannot remain a monk for life, it is better not to ordain. If you disrobe after a few years it will be harder to get a wife, job, etc.” Such words are the unwholesome obstructive kamma of discouraging the wholesome deed of renunciation. Who knows what someone can achieve? Though their resolve may be weak, later they may realise the benefits of the monk's life and the disadvantages of household life, and their resolve will strengthen.
The Burmese idea behind temporary ordination is generally this: “I have family commitments and am deeply immersed in samsāra. Although I cannot renounce for the entire life, I can manage for one week or one month. This wholesome deed of renunciation leads to the accumulation of potential (pāramī).
If my circumstances permit in the future, I will ordain again, or perhaps ordain permanently.” Then if that man's wife wants a divorce, or if she dies, or when the children are grown up, it is much easier for him to become a monk since he has already friends in the Sangha and among faithful lay supporters who would support his ordination.
The Buddha knew beforehand what Devadatta would do, so why did he sanction his ordination? If he knew that Devadatta would create a schism in the Sangha, which only a bhikkhu can do, thus ensuring his rebirth in hell after death, why did he permit Devadatta to go forth? The Buddha realised that by practising as a monk for some years, gaining jhānas and psychic powers, that Devadatta would accumulate sufficient perfections to set a limit to his suffering in samsāra.