The original DMC, when i buiyed and finished downloading the HD Collection on PSN i've started from DMC and finished my first run through normal in barely 4 hours. DMC2 took me roughly the same ammount of time, besides a little more and DMC3 took like 7-8 hours. But all this considering that i've played all the games before on my PS2. But different from DMC3, in 1 and 2 you can kinda "get lost" on the levels as you can backtrack and do that kind of thing, so it can varie depending on circumstances and knowledge about the game itself.
EDIT: I encourage you to buy the HD Collection and play all the games! For a fan of the series even DMC2 is "playable" (it has serious problems with lock-ons and stuff, and the movements are so clunky that for the most time you'll be using your firearms, kinda kill the purpose of combat), it's a bad, awful and broken game but still it's cool to see what Capcom took from DMC2 and reworked very well to implement in DMC3, DMC2 was kinda a "necessary evil" to the series. Oh, and sure: DANTE WEARING DIESEL STUFF, for those who like a more "contemporary Dante".