I have a strange bit about this. My first video was put up in 2007, when monetization was either just starting or wasn't yet offered to everyone. I got a notification in 2008 that my video had passed 20,000 views, and I could put ads on it if I wanted, so I did, just as a lark.
When I started posting videos this year, monetization was available, and I looked at my dashboard and saw that in the 7 years or so since I monetized my first video, it had made something like $15, I figured it'd be fun to see if I could get that money past the threshold to be paid out by Google (the $100 mark), so that's what I did. It took the better part of a year with lots of monetized videos.
I do monetize everything, but I don't expect much out of it. It's such a variable system that I wouldn't want to count on any income from my oddball little hobby.