As often as you feel like putting in make up fluid. The roundy round should saturate the tp roll rather quickly if the fluid is that loaded. It should make many cycles in a short time period, or so I would reason. Just pick a time/mile frame and double it when you start seeing next to nothing in the tp roll. You can, naturally, use the "warm filter" method.
I think some have reported it takes about 3 rolls to get the resident fluid clean.