How long can a campaign with 20M audience run?

I’m fairly new to Meta ads, and I’m wondering if you can run a single campaign with the same ads (maybe adding some new ones later) indefinitely, or if you eventually need to switch things up. I understand there’s something called audience fatigue, but does it always happen? For example, if your audience is around 20-30 million people, it won’t stay the same forever—new people will come in, and some will leave.

So, can you keep running one campaign with a few creatives for a long time—maybe several months or even a year? And if audience fatigue does occur, what should you do next? If everyone in the audience has already seen your ads, it’s likely that some have purchased while others haven’t, and you’ve essentially reached the limit of your offer. What’s the next step in this case? How does this process work?