Ask HN: How long will app income last?

A friend and I had worked on a game which recently took off for us in the Android store, giving us more than $25k monthly in ad revenue on a consistent basis for more than 5 months consecutively.

This is immensely surprising and rewarding for us, of course, but we're uncertain how this income will vary over time.

Does anyone on HN have past experience with mobile game success via ad revenue? How long did the income sustain itself before the game's popularity faded?