Samawood, there is no specific time frame in which the battery can be removed without defaulting the BIOS.  It maybe a few seconds to several minutes.  If you are considering changing the motherboard battery then do it as quickly as possible.

 

As for as backing up your BIOS settings,  there really isn't a need to.  In the event a dead battery for example clears the CMOS it's not a big problem.  When the battery is replaced and the machine booted up, the system will redetect all the installed hardware and set all the BIOS options to their recommened factory settings.  Most of the factory settings you're probably already using.  If you made a personal changes such as the boot order or disabling Fast Boot you will have to go back in to the BIOS and change them.   

 

As for a corrupted BIOS, as was Mombodog explained will require a motherboard replacement.

No way to backup on a Dell, corrupt a bios or a failed flash and your PC is now a boat anchor, or replace the mobo.

 

Once the Cmos battery is removed the cmos is cleared in milliseconds.