Technically you should never have to do detail stripping, but a lot of us do it anyway since it's so easy. If you field strip and clean your gun often you won't have enough crud making its way inside to warrant it. I admit to being lazy and letting my guns gunk up quite a bit before cleaning, which necessitates detail stripping in order to clean everything out good.
Every 3,000 is a good change interval for the springs. Less than that and you're wasting money.