I often start losing fun in a campaign once Absolutism becomes a thing, usually my nation is already the dominating power in the world by that point, with maybe the exception of the green blob, and I just don't see the fun in punching them for the hundreth time as a kind of endboss, on the contrary, those green blobs with their insane force limits are one of the reasons the game stops being fun for me around that time.
In most cases I end the campaign some time between 1650 and 1700 or pretty much just "leave the campaign running" with low level interaction, just to see if something interesting comes around. In my Moldovia into Romania campaign in 1.30 for example Austria turned revolutionary and got a massive coalition, that was really fascinating to watch. They actually won! But such interesting end games are rare.
I'm not a fan of the much more aggressive AI that's been a thing for quite a few versions now. It makes the early game very stressful - if you don't have strong alliances you will be dow'd non-stop - and the end game boring, because you and the AI will have to have blobbed massively to compete.
Diplomacy is also too static to make the mid to late game interesting. Currying favours made it a bit more fast paced, but it still doesn't allow for a diplomatic revolution. Not to mention the problems that "rivalries" cause. Another problem is the Imperialism CB, which makes all other CBs pointless in almost all cases...
In my recent Hawai'i into US game I sat around from 1745 to 1821 waiting for Spain's colonies (all of South America) whom I (all of North America) supported to declare independence...it never happened, so the game pretty much ended in 1745 for me.