Depends on what you use. Google is actually good if you're just getting started and you want to learn about CPC and the eCPM of ads.
Just learn these key phrases (CPM, CPC, eCPM) and then, do google searches from there. You would want to do a bottoms up analysis (for every one active user, I get 12 pageviews, each 1K pageviews serves 50 cents). Then you just graph it out.
There's no good way to estimate it, I'd just go for google ads to start and see what you're getting and graph that out as ALee said.
Your best bet would be to find out how much traffic you're getting each month and attempt to privately sell ads. You can get much higher CPM if you sell private ads versus going through Google.
Just learn these key phrases (CPM, CPC, eCPM) and then, do google searches from there. You would want to do a bottoms up analysis (for every one active user, I get 12 pageviews, each 1K pageviews serves 50 cents). Then you just graph it out.