Not many! I'd say I mastered Mario Kart 64 back in the day though, not in just game completion because that doesn't take long, but in that I could handily beat everyone I knew on 150cc.
I've gotten all the endings on Chrono Trigger and 100% completed the DS version, too, but really with RPGs it doesn't feel like a sense of "mastery" to me, just a practice in patience. The same could be said of most JRPGs I've played, I guess, except I missed some stuff in Xenoblade Chronicles even after putting a few hundred hours into my game. That one's rough, because if you miss just one quest in a couple spots you can't go back to it.