My first long term campaign, in the very early 90s, when we were teenagers in Seattle my brother and I played in Raven McKracken's Synnibarr game and our characters did make it to godhood and adventured a bit beyond that. By any reasonable standard that game reached a satisfying end, though it was open-ended. Raven did a great job making our godquest dramatic and giving us a fine sendoff before we moved back to the East coast.
After that, in college and beyond I played with a couple of different GMs, playing 2nd Ed AD&D and Vampire mostly, with occasionally GURPS or something mixed in, and our completion rate was quite low. One GM friend had high concept games but not a ton of organizational skill and perhaps got overambitious with how many of the (somewhat flaky) crew of friends we played with that he invited, so we were not able to hold any of his games together very long. Six to ten sessions at most, to my recollection.
The other GM was a fountain of creativity, but not all of his concepts really "took" with the players, and sometimes he'd get bored with an idea after a few sessions and come up with another game concept. I think he did run 2-3 multi-year campaigns to completion in that time, but I only played in one of them. And thinking back on it, I don't know if it really "completed" in the sense of coming to a satisfying narrative end for everyone, but it did come to an end and we did think back on it fondly.
In the same era I think we had one abortive game run by another friend, and two deliberate "mini-series" games of 4-6 sessions run by another, which went off fine but I don't think count for a full campaign. So personally, for myself in the 90s, it was two out of dozens.
I didn't play tabletop RPGs for a few years, then some older wargaming buddies wanted to start a game once 3rd ed came out, and in the 3rd-5th ed era my rates have been substantially better.
I ran a short campaign of my own in 3E, but while it did complete the first planned "arc", I wouldn't say it was a complete campaign. My wargaming buddies were more successful, though. Our first 3E game went from 4th level to 20th, and came to a climactic story end fighting demon foes. Our next two 3E games transitioned to 3.5 and went all the way from 1st to 17th or 18th and came to reasonable ends. We had two or three campaigns go for a bit but lose steam and be abandoned when the DM lost motivation, sometimes because of a clash of expectations with rest of the players. Once 4E came out one of that crew ran a game which made it nearly to epic tier but petered out. I also ran one from 1st up into paragon but the group wound up dissolving. For that group, it was closer to 50%.
I had a second group which overlapped that one; we had another 3.5 game run from 1st to about 17th, and came to a satisfying story end. This group wound up semi-melding with the prior group, and had a great run. Another 3.5 game up into the teens and a satisfying conclusion, a 3.5 game another DM ran which didn't work, and then four different 4E games which ran all the way up to Epic conclusions. One a full 1-30 run where we concluded with my Daggermaster/Epic Trickster Rogue slaying Vecna, wielding the Sword of Kas in the Hand of Vecna! The other three started at 11th, but again went the distance, to big finales at or near 30th level. I think I played in two other games in that period which didn't really work out, so the rate for that group and period was 66%.
Since then we've had a 5E game go from 8th to 20th, with suitable dramatic conclusion, and I've played in a 4th ed game my brother ran from 1-10, which went on planned hiatus at a suitable story conclusion. The plan was for a timeskip and pickup again, but one player had to bow out, and we never wound up recruiting another and picking it up again. I'm still going to count that as completed; it had a good run and told a full story. I ran a 5E game for a year and a half which went up to 8th before going on hiatus at the choice of the players, because the player of one of the central/most active characters was going to grad school. We did complete two solid arcs in that game, but had started a third in earnest and were leading up to a big dragon battle, as the PCs sought a cure for a curse they were under in an ancient shrine the dragon had taken over. I'm not counting that one finished. Since that one I've mostly been playing in open-table OSR stuff online, and running a 5TD/B/X mashup for about a year now. Those I don't think really count for this; they're all mostly episodic dungeon delving. Though my game has largely settled into two small steady groups, and I'm leaning toward more plot and continuity for them, so perhaps those guys will get resolution/endings. One of my players has also just recently starting running Rime of the Frostmaiden online, and that's going well so far. So I guess, of those games which would conceivably count toward the score, this latter period is also about 66%?