The turbines at work turn over about 20% of the "sump" volume per annum, which appears to be ample make-up to keep the oil state in the fit for service zone for THAT application. RBOTs look good, and the hydraulics are very clean when we strip them.
But we do have a super duper filtration system (sub 2 micron), and a centrifuge that pulls the water out of the system.
So I'd guess that if you got the particles out, that you'd never need a change.