I say DAY #1! Not monetizing is like walking past dollar bills sitting on the ground and wondering when a good time would be to start bending over and picking them up. It will start s l o w... You will learn, develop and evolve, and it will begin to pay off. I'll never forget when I got my first deposit and thought "I'll be dam'd it really works!"
Also think of this, YT is a Business. They exist to make money, not share your stuff. If you are helping them make money, they will be more likely to promote your stuff. The more people you attract to your stuff so they can see and click on ads the more promotion YT will give you. Make no mistake, this is about MONEY and that's ok.