If you grew up in the 90s or early 2000s and played PC games, you've undoubtedly heard of the Heroes of Might and Magic franchise - a series of turn-based strategy games by New World Computing. Heroes of Might and Magic 2 was the first truly amazing game of the series, followed by HOMM3 with improved graphics, and then going to crap starting with the 4th one. I think HOMM2 and HOMM3 collectively stole not months, if not years, of my time, and I am still just as excited to play either of them as I was back in the day - the replayability factor of these games is through the roof.
When I was growing up in the 90s, one of my favorite games I probably spent a combined year of playing time on was Heroes Of Might And Magic, a turn-based strategy that you can play for weeks. Since each move lasts as long as you want, it's easy to keep playing whenever you have a free moment (or a block of 5 hours in my case), and the vast number of strategies, heroes, units, and resources provided for almost infinite replay value.