Title says it all folks, I've tried researching the respawn times of naturally growing alchemy ingredients e.g Tundra Cotton, Deathbell etc, but nobody seems to know for sure :S...

I DO know that it's not 3 in-game days though

Any help would be greatly appreciated.

P.S I'm on 360

I think it's 10 days for the default respawn. 30 days for the "cleared" areas.
But be aware that this is if you DO NOT visit that cell in that period. If you enter the area, the timer resets.