just received my new 24" iMac. How long does it take to charge the magic mouse.

Welcome.

Most rechargeable Apple gadgets ship with a ±50 percent charge.

Are the mouse and keyboard plugged into the computer's USB ports? If so they should be charging.

I have to ask: Did you find the tiny "on/off" switches on the two devices? I'm not trying to being snarky here; many people are not accustomed to keyboards and mice having power switches, and not seeing the switches id a common report here.

2 min charge =~9 hours

120min charge =~30 days

This all depends how long you use it on average.