Mainstream (for the time) - Star Wars Galaxies - Approximately 8000 Hours.
Porn Game - Probably Man of the House by Faerun. I was a stubborn tool and wouldn't actually use the built in guide
The one i have played the longest would be "Skyrim".
If it is a porn game i am the same as you, it would be the option since i have lost my saves a couple of times "Peasant Quest"
Main: Guess of all time is Minecraft. Bought it a very long time ago before it was big for 5 bucks. Now I have thousands of ours in it and own it on many platforms
Porn: I'd have to guess Single Again.
Gwent The Witcher card game (3000 hours), Dota 2 (2500hours), Elden Ring (800 hours) for non porn games.
Parental Love, The Visit, Sisterly Lust, these aren't long games but I've replayed them a fair bit.
main: I might show my age a bit, but l4d2 was at least 500 hours. The creativity and and community around it was great. 10 survivors v 10 special infected was chaotic and amazing.
XXX; Honey select 1/2, the customization and features were amazing. RIP to the studio shutting down