Most are set to 3 - 6 months as I recall. I'm pretty sure it's coded in the engine. The shop contents for all shops are stored at the end of the save.HDR file. That's the only way to keep items separated for different saves. The Frostgard shops got left out so they restock every time you load a save. Makes it easy to buy anything you want.
I'm sure area respawn dates are stored in there too. I never bothered to look into it since I never take more that 33 days to play a game (after the first time through).