for me its probably something along the lines of,
DMC:4hours
DMC2:not worth finishing
DMC3:6hours
and thats including finding all the blue orbs but not the secret missions..if u include the secret missions u can add about an hour to the time...they have been pretty easy thus far..but mind u thats after my first time playing..meaning my first time it probably took alittle over 3 days for each game..lol...however 4 is taking me forever....ive had it since the day after it came out and still am only on mission 15