At what point does a game's length becomes an issue?

[StokedUp]

I've noticed a pattern with myself, I play mostly RPGs but I've noticed once it hits the 70th hour (give or take) my playtimes start to go down as the game starts failing to keep me gripped. Usually if it hits the 90th hour and it still isn't done and there's another game out I'm interested in then I'll simply stop playing and move onto another game (I'll end up coming back to the previous game to finish it at a later point).
I think the only Rpg that's kept me going past that it Takes Of Graces F, in which I played for 140 hrs before finishing.