When 40% of your pilot customers insist on continuing to use your product you know you have something.

When 40% of the visitors to your MVP click on the "by now button" only to find out it doesn't work it is time to monetize.

When 40% of the people you show your paper based wireframes say "I buy that and will even give you a preorder"

Ideas are worthless.
How you comes from execution.
Work with early potential customers let them be your guide to monetization.