How Long Is A Good Campaign?

Like the title says, I'm looking for opinions on how long a campaign ought to be. I'm currently working on stuff to prep my homebrew world for another play-test, and the last one was about 36 hours of play (plenty of RP, of course, so that number could vary somewhat wildly for others) and a dozen or so quests and another dozen Points of Interest. That got them to around level 8 or 9 using milestone levelling.

Problem is, as I continue to add to the world, I'm...not sure if I'm making it too big? Like, I want the world to feel full, and its meant to be a 1-20 campaign with post 20 content. I currently have 30 quests/Points of Interest, with plans for at least another ten.

Is that too much for a single-party campaign? Should I have important quests for single parties, but have extra stuff for people who want to do West March-style play?