Personally I think it'd take quite a while to completely exhaust my interest, just because it's so open-ended and there are many different ways to tackle certain things. For example, I typically don't use a lot of animals, only 1 or 2 of the critical ones and that's all, but I'm recently playing a save in a Meadowlands farm wherein I went the opposite way and have tons of animals (4 barns and 2 coops full and counting ... that may not sound like a lot to some but for me it's a LOT

).
There's also remixed bundles to spice things up, different spouses to see heart events for, etc. I guess it depends on what it is about the game you enjoy (I once challenged myself to catch a very difficult fish with challenge bait ... but that didn't actually work out

).
I do run out of steam occasionally and put it down for a while, but then the interest will come back and I'll start it up again (maybe I'll be bold enough to try the beach farm at some point... seems challenging...)