I think my highest total is in Diamond, with around 450 hours. I think I spent a lot of time training, completing my Pokedex, trading, etc. I might also have fallen asleep with it on a couple of times, as I'm surprised that the counter is that high.
As for the fastest rate of adding time to a game, I have about 230 hours on X. I used to play much more frequently before the semester started, but since the beginning of February, I think I've only put on about 30 hours.