Right from the game's outset you'll be aware that a helicopter is going to drop by the mall and save you after 72 hours. In the meantime, it's your job to find out what you can, save hapless humans and make your way to the rendezvous point before the timer runs out. Though the game tracks your time, that doesn't mean it takes an actual 72 hours to finish. According to Capcom's Heiji Inafune who's working with both Dead Rising and Lost Planet, the game will take somewhere around two, possibly three hours to beat if you just stood still and didn't attempt any sidequests. Yes, you read that correctly.
While the game's staggeringly short length may surprise some, Capcom designed Dead Rising to be played through many, many times. The ultimate goal of the game is to discover the secret of the zombies and save as many people as possible, though you're in no way required to do so. Designed to be totally open-ended, Dead Rising allows you to investigate whatever you want, kill whoever you want and finish the game by completing whatever objectives you saw fit. Despite the hilarity of peppering innocent humans with shotgun blasts, you're not going to get the best ending without at least helping some of them. Many of the side quests yield information important to your investigation, and killing certain foes will be the only way to access areas you'll need to visit to get the answers you seek. Figuring out how to amass all the clues and get the best ending is the core gameplay mechanic of Dead Rising, and what should keep you slaughtering zombies over and over again.