Rockstar Games have said the main storyline for L.A. Noire is between 25 to 30 hours in length, but there are always ”unassigned cases” to pick-up.
There are 23 main ”assigned cases” Detective Cole must work through. L.A. Noire supports new cases through DLC as well - some are pre-order bonuses.
L.A. Noire is a detective game built using a new state-of-the-art animation capture technology that gives the eerie appearance of ‘real-life’ performances in the game, which is crucial because as Cole we need to weed out lying suspects.
L.A. Noire releases on Xbox 360 and PS3 in US May 17th, in EU the 20th.