Just beat Dead Rising and got ending A then beat over time mode.
How long is the game? 15-20hrs? Do you get the A ending by getting all the scoops? 
Yet another game on the shelf that I need to make time for. 

Shorter I'd say 10 to 12 hours (though I wasn't paying attention).  Though I can sit around and play a practically endless amount of time just running around killing zombies. 
As far as the endings go: 
Ending A   Solve all cases, talk to Isabella at 10am, and return to the heleport at 12 pm on the 22nd
Ending B   Don't solve all cases and be at the heliport on time
Ending C   Solve all cases but do not talk to Isabella at 10am on the last day
Ending D   Be a prisoner of the special forces at 12 pm on the 22nd
Ending E   Dont be at the helliport and dont solve all cases
Ending F   Fail to collect all of Carlitos bombs on act 7-2