That's the question, isn't it? When you get to my age, you realize the two things - what you'd ideally wish for an RPG campaign, and what's likely to actually happen - are entirely different. And you learn to prepare for the latter rather than the former.Ideally or what actually happens?
Normally they finish around level 8-12. Last one died at level 9 due to lockdown.
I would ideally like D&D campaigns to last two years, or from level 1 to 14. In practice, they tend to last about 8 months, or until level 7 or so at the rate we play.