that is a wide open question, sort of like what rpm does an engine need to drive up a hill? what kind of engine, how step a hill, etc
when i was looking into trying to build my own years ago it seemed to break down into centrifugal or centripetal force. the greater the measure of gravitates or g's of force you could place on the liquid to a certain extent the faster the contaminants would fall out of the liquid. a centrifuge is basically doing gravity filtering in a greatly accelerated state.
i did find a recommended g's of force the liquid should reach to drop out particles. i also found some online calculators to show the relationship between rpm and drum diameter in how much force you are creating. basically the larger the drum the slower the speed to reach the same g force.
it was cool to play with some calculations and you really have to start watching parts design, strength and balance so you don't make a centrifugal explosion. in the end i just picked up a ready made option from pabiodiesel.