The optimal g will depend on what you're trying to remove from your
bacterial culture. I'm a cellular slime mold person, and we use
an 800 x g spin (about 10 minutes) to pellet slime mold myxamoebae
while leaving the bacteria they eat (E. coli, in our case) in the
supernatant. If your contaminant is about the size of a yeast cell,
this should work well. If it's smaller, a faster spin may be in order.
(Slime mold herder, Department of Biology, RPI)