I don't have any hard rules about when I allow myself to become more lax about the ferm temps, but here are some of the things I look for:
-less requirement for cooling
-less chunks floating around
-a layer of yeast trub beginning to settle
For an average gravity beer with a proper amount of yeast pitched, all these factors mean I wait 2-3 days until adjusting temps. Until then, I try to keep it in the sweet spot. I don't really take gravity readings during fermentation, but I would guess all of these signs indicate that fermentation is at least 50% complete, and probably closer to 75%.
I also make sure that there still is some yeast activity going on when I stop controlling temps, because I want the temp to slightly raise. This keeps the yeast active and they clean up any by products and fully attenuate.