That decision is often made for you by the nature of the contract on offer. In general terms, the longer you can work abroad the more useful you will become. Unless you go for a reasonable length of time, the difficulties of adjusting to the new situation and 'learning the ropes' may outweigh any benefit to be gained by those you are going to help. On average, you start to become professionally 'useful' after 6 months and should have achieved a reasonable grasp of language and culture by the end of your second year. However, the longer you stay, the more difficult it may become to maintain revalidation and re-entry to the UK may prove to be more difficult.