I have a feeling that a lot of people are going to want to try this, so I'm posting it here to give people the chance to just mess around in 1 game for as long as they want without restarting it. I take no credit for this glitch, and as of this post I have not tried it but other people have and it seems to work. Here is the procedure:
 
Source:  http://www.gamefaqs.com/boards/989132-dead-rising-2-case-zero/56224076

1. Start game go to ambulance and retrieve Zombrex
2. Waist time around till it tells you to give Katey Zombrex and give it to her.
3. Once that's all set run back and get the bike and take it to the front of the safe house. BUT NOT CLOSE WHERE IT TRIGGERS THE BIKE CUT-SCENE!!!!!!
4. Sit outside and wait till you have 1min left before the military arrives then quickly take the bike and bring it inside. Let the cut-scene play then the glitch is active.
5. leave the safe room and it should say the military has arrived but nothing will happen 

If this method doesn't work look for a variant in the thread. Basically the method seems to be that you give Katey her Zombrex but do not return the bike to the garage when it gives you the cart. You have to wait until the last minute to do it. I don't believe survivors will spawn, or anything. The OP gives a fair warning about what you can and can't do.

Just do not return to the garage until at least 12:00 am 9/13 or your game will end. After this time , you should be able to return to the garage whenever you want.I always save outside the garage anyway. Also, when you save your game, it will say FAILED.

Here is a video showing the glitch: (please note that in this instance there are no zombies in the infinite time glitch, which can happen sometimes)
 
 

" 2. Waist time around till it tells you to give Katey Zombrex and give it to her.    " 
 
Tell me, how does one "waist" time? Cause I only know how to "waste" time myself.

As it turns out Dead Rising minus the Zombies = not much fun.

" As it turns out Dead Rising minus the Zombies = not much fun. "

Do you reckon you can do this and still save the survivors? That'd make getting the achievement a lot easier.

Ah, that does make sense. I'll still try this, but I guess I'm gonna have to get the achievements the old fashioned, self-punishing way.

OK, I just tried the glitch and it works. When you deliver the bike and exit it should be 8:59 and when you exit it says the military has arrived, you must escape but you are outside with no objectives. Time to explore!

Hmmm. I figured more people would be interested in this. Just a quick update, I've been playing for over a day past the actual events of Case Zero and no negative effects so far. Katey and the bike have gone, no objectives, no survivors, no shed key but everything else is fine. Just killing zombies. Finding every weapon I possibly can and using it, grinding for money (to get the $100,000 achievement). I can even go back to the gas station now since it's been over 12 hours so the world gets refreshed (including all the slot machines) and can build boomsticks and chainsaw paddles until I get bored.

" Hmmm. I figured more people would be interested in this. Just a quick update, I've been playing for over a day past the actual events of Case Zero and no negative effects so far. Katey and the bike have gone, no objectives, no survivors, no shed key but everything else is fine. Just killing zombies. Finding every weapon I possibly can and using it, grinding for money (to get the $100,000 achievement). I can even go back to the gas station now since it's been over 12 hours so the world gets refreshed (including all the slot machines) and can build boomsticks and chainsaw paddles until I get bored. "

Another update - Don't save and load in the gas station otherwise all the zombies will in fact disappear. Just save elsewhere if you must.

" " 2. Waist time around till it tells you to give Katey Zombrex and give it to her.    "  Tell me, how does one "waist" time? Cause I only know how to "waste" time myself. "

The town doesn't seem big enough for this to be entertaining at all, unless you just want to kill zombies forever

Yeah, not much point to this one.  It's more than easy enough to get the 1000 kills achievement without it, and if you can't rescue survivors, there's not much else to do.  I sorta doubt that I'd want to spend any more than the hour or two the game takes farting around before everything got boring.  About the only thing I'd wonder about is does the clock keep going, and does the sun come up in the morning?

Can you save afterward?

This is awesome, I can't wait to try this out.