Generally, a couple of inches up or down at most before you have to get into recutting (for the down). The limit on the alteration in the out direction is how much allowance there is in the seam. In the in direction, it's not having the back pockets touch or look silly. If you are going in much, the pocket balance can be thrown off, so they start thinking about recutting the pockets in front, etc.
I was positive there was a comprehensive topic on this, but I couldn't find it in a search.