That's the sign of a good map - repeatability. I think I've played four separate saves on Oakfield Farm across FS17 and 19 (three in 17, one in 19). Similar with Coldborough Park Farm across FS15 and 17.
I usually get bored of a map when I've upgraded to big machinery, but if I'm drawn back I'll usually play a different style or location.
I can't say I've ever looked at how long I've played a map for though, even though it's right there in front of you on the game saves page. I just stop when I stop enjoying it and length doesn't come into it.