I think this is something on which each person can trust their own intuition. Some people know that they tend to need a little bit of time — weeks, months — in order to read compatibility and interest accurately. Other people assess that very quickly and aren't wrong in their assessments. Still other times, it depends on they dynamic of both of the people involved.

But whenever your gut is telling you, "I can't be happy with this person on the level that I know I am capable of being happy"... then that's a perfectly valid time to stop wasting their time and move on to someone else... or just be single. :) (At least that's just how I see it.)