Nordic larp designer Claus Raasted has a listicle (because Medium) on campaign formats:
NZ GMs have tended to avoid open-ended campaigns and gone with a shorter, 2 year format. This avoids the worst of long-term power creep, while allowing mechanics to be tested and evolve. That said, we could go shorter (Kingdom was three episodes, and felt satisfying, and there’s definitely space for large one-off events).
He’s got a definite point about campaigns as a recruitment tool: campaigns are great at getting people hooked and getting them to stick around. and Crucible (the biggest of them, which died partway through a few years back) brought a hell of a lot of people into the larp community. The way to grow larp in a city is probably to make sure there’s always a campaign or two to choose from.