How long does it take to replace bottom bracket bearings?
There are three main categories of bottom bracket, adjustable type, cartridge-style, and external-bearing. 

Cartridge-style bottom brackets are not serviceable, the replacement requires replacing the entire bottom bracket assembly. Removing the cranks and installing the new bottom bracket is a pretty straightforward task, and should take less than 20-30 minutes.

Adjustable-type can require a little bit of time to get the tension right, plus they are typically more difficult to remove due to the fidgetiness of lockring spanners and the fact that the fixed-cup is typically seized hard into the frame.  I'd allot ~45 minutes to an hour for that task. Depending on the brand of cranks.

External-bearing splits the difference - some have more steps  than cartridge bottom brackets, some are equally simple.

Note that in all cases, using the correct tools is important.