One quarter. I used to do 20 all the time. To graduate in time to get a gig (that vanished when the company went into a hiring freeze) I did 30. My wife, when pursuing her doctorate, never dipped below 28. But yeah. Typical quarter load is 14-16.
Typical semester degree is 120 credit hours, right? Typical quarter degree is 180. Across 4 years that's 12 quarters is fifteen credits per quarter. 120 credit hours across 4 years is 8 semesters is also fifteen credits per quarter. 26 credits is 16 credits (full load) at one school and 10 credits (2/3rds load) at another school with a 30-minute commute between the two. it isn't quite two semesters at once but it's close. 30 credits a quarter is two semesters at once. And I've done that. And my wife made a habit of it. And it still took her five years.
If it's a real class it's fairly fucking insane. Fortunately this was 26 credits of votech so it's mostly just time consuming. But it hasn't been atypical to drive 45 minutes, spend 5hours on my feet operating a CNC mill, drive half an hour, spend 3 hours on my feet using a jeweler's torch, drive 45 minutes, make dinner for the family and then spend 3 hours hunched over a workbench disassembling and reassembling a pocketwatch.