I regularly rack up 300+ hours counted in each Pokemon game.
Now, that's not to say I actually spent every hour of that time playing. A lot of the time in my White version (281:30 and counting) was spent with the DS just sitting open because I had forgotten about it.
Of course, there's also the grinding in between gyms/the Elite Four, the Battle Tower/Frontier/Subway, exploring post-game stuff, and just getting bored and going out to catch Pokemon...
For me, it's pretty easy to rack that kind of time up.