It depends on so much. Barring all reads, if i'm heads up in a pot, and i completely miss, no pair no draw no idea if opp is laggy or tight or whatever...i will fold to just about anything. 1/3rd of the pot would take it. But do that 5 times, and eventually i'm gonna push. Or eventually i'm gonna make the hand and sucker you into another bet or two. Its hard to make a general rule for it.