I stopped tracking it for my players after 7th, I just milestone them when I think it's time.I like campaigns that go the amount of time I intend them to go.
My D&D includes:
• One Shots. A one to three session adventure unconnected to anything else, generally with characters used (or leveled) only for this purpose.
• Theme Adventures. An focused, thematic adventure designed to run somewhere between 8-15 sessions, with character custom created for this purpose (and who generally will not level up during the adventure).
• Mini-Campaigns. The best example of this is Lost Mine of Phandelver. A few months of play where organized around a large adventure or series of adventures, freeform or more structured, where the characters should advance multiple levels.
• Campaigns. A multi-year (the longer the better) exploration of the lives of the PCs, as they advance from their starting level towards an expected end point, generally at a much higher level than the starting level, and which may include any number of unrelated adventures and adventure arcs in addition to any that may be related. There need not (but can) be any story or event focus other than following the lives of the PCs.
I don't really consider anything that lasts less than 4 years a true campaign. If it were intended to be a campaign as I defined it, but stopped before hitting the intended end, I consider that a failed campaign. I'd honestly rather have played something else than to have started a D&D thing (whichever of those it may be, but especially one intended as a campaign) and have it not get to its completion.
I know it seems to be a pretty common thing for people to start a lot of campaigns and not finish them, but in my opinion that is a planning failure half the time (the other half of the time it's someone else's fault). I think the problem is that people really don't think about where they want things to go, and how they plan to get there before they get started. Kind of like (to bounce off of something from the OP) a lot of TV series. Some TV series are well-planned, they tell the story they want to tell in the right number of seasons, and then they end it in a pleasing manner. They knew what they were doing before they started, and it shows. Other TV series just sort of bumble along after an initial good season, without any real purpose, getting loony and stupid until they are eventually canceled. Don't let your campaign be that type. Be the planned type.
Now, out of that list I gave, my absolute favorite is the true long campaign. "Sim-Adventurer", so to speak. Planning doesn't have to mean plot. The plan can easily be, "The PCs adventure until they reach about 20th level, and then retire rich and well-settled in the world. Oh, and hopefully they'll kill an evil god or found a kingdom in the meantime." In fact, that's pretty much the old school D&D campaign goal.
But if I'm doing something other than a long campaign, I figure out what the heck it is going to be first (generally from that list I gave above) so that I can make sure it happens in a satisfying manner. Otherwise, you end up with failed experiences rather than successes.
(One thing I should point out is that it practically impossible to run a real campaign in 5e (or 3e, or 4e) using the default XP rates, because they are designed to level your characters way too fast.)
Sent from my SAMSUNG-SM-N920A using EN World mobile app