Holy crap. I think the only game I've played through in one sitting is CV1. I've tried CV3 twice, and by the time I get to Drac, I just don't care anymore. I know I've beaten him countless times in the past, and I know I can do it again, so I try it a few times and quit...it's just not worth the effort after several hours of gameplay. Of course, I usually play through the games with friends, and it goes kinda like this: I beat a stage, they try the next stage and die 3 times, I beat that stage, they try the next stage and die 3 times, rinse, repeat. So it takes awhile. My buddy and I did take down Simon's Quest in two sittings a few weeks back, which I thought was a decent accomplishment.
Then again, I only have time to play about 2 hours of gaming a week, give or take.
Wow, I could never do that. I usually play a video game for about 1 hour or 1 hour and a half at a time. If the game is really addictive I might return to it 2-3 times in one day, but I could never ever play for that many hours in a row. When I was younger things were quite different though. For example, I think I finished Shining Force II in 5 sittings, which is kinda extreme, at least for me.
My ideal Castlevania Marathon would be hooking up all my old systems and play a little bit of everything. I wouldn't try to finish each individual game, just play a few levels for fun
Not the Castleroids though - I don't wanna start playing them if I don't have the intention of beating them. And I don't want beat any of them ever again, at least not in a very long time. The oldschool vanias are fine for a quick romp though 
^This, to the letter.