Back when the iPhone Developer Program was first announced, developer provisions (the 'permission slips' that allow developers to distribute pre-release builds of apps in progress) lasted one year. It seemed natural to have a one year expiration, as our developer memberships also lasted one year.
Everything was all fine, developers created new provision profiles as they grew, and each lasted one year. However, sometime in May of this year, provision profiles seemed to start expiring after 90 days. At first, many thought this was linked to the expiration time of their iPhone developer memberships, which would decrease the time to use a provision.
However, it seems that it's been set that provisions are only going to last 90 days. Also, distribution provision profiles, which are needed to submit applications to the App Store or distribute applications via ad-hoc, now only last about six months instead of one year.
If your provisions are expiring, your iPhone will remind you to renew your provision, and will state when that provision will expire.
If this is the way it's going to be, we may have to live with it -- it's just something that I would like to stay consistent, rather than wondering every time I renew a provision whether Apple has swapped out its stopwatch again.