If you know what you are doing. Probably 2 to 5 hours. If you are just going through it casually, 10 to 20. This is because you might have to repeat certain areas as you are unable to get past a trap or a boss. Good game, but they are designed to be like roller coaster rides. Intense but quick. Now if they made you repeat the same level over and over again in order to gain experience points. Then it would take about 50 hours like an RPG.
1 ~ 8-10hours 2 ~ 10-15hours for the story, +~30-40hours with extra content. 3 ~ 15-20hours for the story. +~8-10hours with Metal Gear 1 and 2(which comes free with it) 4 ~ 15-25hours. Peace Walker ~ 8-10hours for the story. +~70-100hours for ALL the extra content. Rising ~ 8-10hours
when it comes to MGS 3 and 4 the times can vary wildly depending on how you play.
Currently playing: Disgaea 4 - Resident Evil 5 - Mark of the Ninja
One and two can be finished in roughly two hours once you know what to do. A first time through them will last far longer though. Three and four are much longer even when you know what to do. Peacewalker is a different beast.