I don't know about most of the campaigns that I've been involved in, but for the last two that I've run, I have kept session logs.
The campaign previous to the current one ended after 73 sessions. Characters has just reached level 10. A couple of 'story arcs' had been completed, but several more were still in progress when the campaign ended due to the death of a player.
The current campaign just had its 64th session. Characters have just reached level 9. Three 'story arcs' are complete, one is very close to completion, two more are maybe a third to half-way through, and three more are one-quarter or less complete. An uncountable number of other possible story arcs have been passed by or discarded by the player characters (or the players).
By 'story arc' I mean a problem presented to the player characters that will take multiple sessions to solve, like, "The Draconian Empire is about to declare war on the planet that some of you y'all are from. Keep your home planet from getting wrecked." which was resolved via the player characters finding an alternate food source for the Draconian Empire (among several other solutions or possible solutions that were attempted or partially attempted and discarded) or "A mysterious crime family is gobbling up all the smaller crime syndicates in the sector. Find out what the eff is going on." which was resolved by the eventual taking over of the mysterious crime syndicate by one of the player characters (after going to 'work for' the mysterious crime syndicate and having the crime boss attempt to kill the player characters several times in an elaborate triple cross scheme between the player characters, a couple of planetary governments, and the crime syndicate). Or a goal that the player characters decided to pursue, like, "develop a faster than light communications network so that we can call each other when in different planetary systems." which also certainly helped resolve some of the other things or, "open an academy to train young psions to use their powers so they can defend themselves against the depredations of the Draconian Empire (and other forces that the player characters didn't know about at the time) which ended badly, but opened up another, even more revengier story arc.