My thoughts on this matter were summed up today by Marc Andreessen:

In my view, entrepreneurial judgment is the ability to tell the difference between a situation that's not working but persistence and iteration will ultimately prove it out, versus a situation that's not working and additional effort is a destructive waste of time and radical change is necessary.

I don't believe there are any good rules for being able to tell the difference between the two. Which is one of the main reasons starting a company is so hard.