1/4" of crud is too much for me. I let mine run until I get no more crud under the bottom plate and passages to the jets on the rotor. Maybe it's my OCD acting up, but I like it nice and clean. How long does that take? As long as it takes. It depends on how dirty the oil is. Time and gravity are your friends. I have some ***** tanks that I pour the latest WMO in. I'll let it sit years! and then pump from 6" above the bottom of the barrel. It then gets boiled to remove the water. It's surprising how much water there is in it. I only boil about 2/3 of a barrel, roughly about 140 litres. There needs to be enough room in the barrel for the oil to boil, and the oil expands as it heats. It is very noticeable!
It then gets mixed with 50% diesel and then another 10% biodiesel. Then I centrifuge it. The thinner the mix, the better the crud comes out.
I run my mix in my 7.3 PSDs. Even in the Project Parts Truck. They like it. The engine seems to quite down and make more power.
The mix of high cetane fuel (biodiesel rating of 50) and low cetane fuel (WMO rating of ~40) works well. I suspect the biodiesel ignites earlier and helps the WMO burn nicely later when the crank angle is more favorable.
I use a batch system rather than a continuous run system. I never leave it unattended. Too many close calls.
It's bad enough having to clean up a pint of oil, let alone having to deal with a whole barrel of oil/fuel.
