For example, if I spent 6 months on an app, I doubt it would pay rent, because there's a lot I'd have to learn and it would probably (?) be amateurish. On the other hand, if I cut out 18 months and kept iterating, I'm sure I'd do better, financially, than I'm doing at my day job.
I haven't won this game yet (I have a fairly typical day job for a top-5% programmer of my age) but it seems like the trick is to find an avenue where you can rapidly get better, invest heavily in yourself when you find it, and eventually be independent.