I've seen systems that could go for up to 30 mins without the battery. I've found the quickest way if you don't know what jumpers to use (or if the !@#$%%^ board doesnt' have pins soddered to the contacts like my ASUS A7M266-D) is to remove the battery and then touch a pair of pliers to the 2 contacts that the battery uses. That will short out whatever charge is left in the system and instantly reset the bios.
Please not that when you do this, the bios will revert to default settings everywhere, so you will have to make any changes that had been made before (ie configuring HDD manually, changing boot order, numlock on, power management settings, etc)
Also, be sure to ground yourself before doing then and UNPLUG THE COMPUTER