How long do I run an experiment for?

A great, actionable guide I'd recommend is the book Running Lean.

But knowing is half the battle. By this point everyone should know about Lean Startup and Customer Development. The true test is having the discipline to apply the principles rigorously and not cut any corners. That's where most entrepreneurs stumble, I think.

Test the idea until you either find a result (good idea or bad) or you stress your limits (financial/emotional).

Read "Why Experiments End", which addresses exactly this question.

Did you mean "How Experiments End" by Peter Galison? It looks perfect for helping answer this question.