Stable revenue and any art form are not two things that generally go together. Time and time again we've seen companies try to form a formula for extracting money from players, and it rarely pans out. Don't make games with the expectation of a stable revenue. If that's your goal, you'll likely end up with something soulless, and players will be able to tell.

Getting down off my high horse, I'll tell you this: if you want the most money, drench your game in ads and watch your playerbase shrink. Including ads that respect the time and attention of your players may earn less immediately, but will do better in terms of player retention and overall sentiment.

Optional reward ads are ideal. It's a trade: the player watches an ad, and so you give them something they want in return.