50% for long-form campaigns - I'm not counting the ton of 1-4 session short games I've run. I ran a campaign back in college and one in grad school that didn't finish. My more recent two campaigns - a 5e levels 3-20 game that went for 3.5 years, and a 13th Age levels 1-9 game that went for a bit over 2 years - both finished successfully. They didn't have pre-defined endings, but they came to a point that closed out the events of the campaign well. I'll admit I blitzed through the final few levels in both though - could have ended at an earlier level, but wanted the players to get to use some of those cool high-level powers of theirs (and in the 5e game, the level gain was also being imposed by an in-world force and serving as a timer to the characters exploding).