I would say anywhere from six months to 3 years for most games to fully play out whatever idea staff started with. After three years, it feels like most games get a) bloated with PCs that are too powerful and entrenched to be adequately challenged without upending the board, and b) the plot is either completed, or has been dragged out so slow that it’s hard to stay engaged with.
I sort of wish more games went in with a campaign mindset of ‘this is the story we’ll tell’ and when that’s done, either tell a different story with different characters in the same setting (timeskip, sure, but also could just be on a different part of the continent, or looking a different faction of characters, etc. - I’d love to see a game, for instance, that told the story of two wars with one as the PCs as the heroes of one side, and the next with the PCs as the heroes of the other side) or look for a different story and setting entirely.