It depends on the enviornment. Most will say 3/4 capped (ref. Beekeeping for Dummies), however, given the humid enviornment down here, I try to make sure that at least half of the frames are completely capped. I think anywhere from 3/4 to completely capped is safe to eat. Just, don't harvest when it's any less rippened. Unripened honey seem to be a natural laxitive.