i don't remember the end-game of the original Diablo; however, in diablo III and IV everyone i've tried to play the game gets bored in the run up to max level. I always tell them "i skip that part as much as possible, because that's not the game. That's just the story!"

Once you hit max level in III and IV, the game actually "begins."

and to explain the Diablo 2 Reference, the amount of time/effort it takes to go from level 98 to level 99 (the max level), is the same amount of time it takes to go from level 1 to level 98. I've heard "2 weeks" as a rough estimate of "unhealthy playtime", at least solo.