Anything from 10 to 50 gallons - I have a lot of spare tanks, some of the smaller are always running (as daveeka's are) as snail production tanks, plant offset grow-out, or fry grow-out. Larger tanks as required are available not in service, and spare filters are always running somewhere. A few hour's warning and I have anything in the range operating with mature filters and aged water.
But usually new specimens are going into species tanks, so their tank is set and established and ammonia-challenged to be sure prior to the new fish coming home. Then their permanent home is labeled QT for the first 4-6 weeks, or until I am sure they are clean.