In short, Microsoft offers "mainstream" support for OS releases for about five years, with "extended" support for a few more after that.
For example, Microsoft will continue to support CE 3.0 fully till June of this year, then will cover it under extended support for three more years. After that period, Microsoft will only offer online support and suggests that non-Microsoft sources like partners and online chats would be resources after that.
Generally, devices that stand alone and don't use external media probably don't have to be concerned with OS updates. We have seen that the evolution of removable storage media and CompactFlash/PCMCIA hardware are often what drive the need for applications upgrade to the latest OS builds.