I am sheepish, because even though I have been talking about it for ages; I still have not organized a toy rotation. But I think depending on how much you have out, around 4-8 weeks is good. If you are only trading out one or two things at a time, maybe you can do that every 2 weeks or so? That way it's not a whole huge changeover, but one or two items, swap out the stuff that's been out the longest, etc.