If it's SaaS, you should be monetizing from day one. An MVP in SaaS is the minimal product that people will pay for, that's the starting point. You evolved the product from there.
If it's a consumer app, that's a little more complicated. Often times, monetizing preemptively can stunt growth and kill the trajectory, due to lack of focus, user experience, etc. It's really a question of access to capital in this scenario.
We (as business owners) do not determine when an idea has value.
The only thing that truly can make that determination is the marketplace.
Do market research and determine if there is a sizable enough market of people willing to pay for your product/service. If there is then it is able to be monetized, if not it's not.
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.