Well you shouldnt really stop until cease making progress in your lifts. So if you keep adding weight each week, and keep getting all your 5x5 then stick with it. And if you dont get it one week, dont sweat it, give it another go the following week at the same weight and you might surprise yourself. If you truly plateau, then there are variations you can implement to help break that plateau (ie. doing a 3x5 or a 5x3). I have played around in the past with switching the compound movement completely after exhausting all other possible variations and it was quite effective, however, I would give everything else a try before you decide to switch the movement entirely. Hope that helps, Im sure other 5x5'ers will chime in as well.