There are two types of exit permit. The type good for one use, which expires if not used and and unlimited exit permit which is good for one year. You should ask for the unlimited kind, if your sponsor is amenable, which will avoid the hassle of having to get one each time you leave the country(there is also a fee charged for the unlimited type), but don't be surprised if your sponsor refuses. There is no penalty for failing to use either type of exit permit.