I once played Ogame during summer break, but quit it midway through the break when I realized I was trying to time at least some RL activities around game timers. I'm glad I learned my lesson after a month and a half or less, though.
Back in the days when the DS was new-ish I remember beating a thousand or so old axe armors in Castlevania Portrait of Ruin to unlock old axe mode, which eventually led to the "strategy" of going to an enemy rush dungeon, clearing a room with several of them in it, toggling sleep mode on and off so that I'd respawn back in the same room with respawned enemies and repeating the process many, many times. Man, that stuff sure was mind-boggingly repetitive and dumb as hell, but the extra game mode was worth it in the end, IMO.
But the cake probably goes to EV training pokémon. Beating hundreds of pokémon just to give yours some extra stats is still quite tedious nowadays, but back when all you had was Pokérus to boost your EV gains and wild pokémon often gave you one or two EVs at most it was an even bigger pain in the ass. Breeding mons was also boring, but alternating two directions was easy enough to do that you could be doing something else in the meantime, compared to having to find and battle hundreds of underpowered mons over and over just to get 1/4 of a stat point at level 100 at a time.