I know it obviously depends on how much 100LL they go through, but how often does the average FBO get a new supply of 100LL?
A big "it depends". Where I worked we had 1 large 100LL tank and kept all our 100LL trucks full, even the crappy "only for fire season" trucks. I think that gave us around 18,000 gallons of 100LL. We'd get a load of AvGas every couple weeks which was 8,000 gallons. We tried to keep full in the summer because of the P2's came in they would take 1,000 gallons at a time, with 2-3 of them there fueling 3 times each a day... yeah you can see how that would go. On a normal summer day we'd sell 4-700 gallons of it. During the winter... maybe 200.
It took 2-3 days from ordering 100LL to receive it though, it is not made very near by.
Jet fuel though.. in the summer we'd sell 15,000 gallons a day, during the peak week we could see 20,000. Receiving 2-3 loads of Jet in a day (11,000 gallons each) was not unusual just to keep up with demand. Fortunately the refinery for Jet was just 2 hours away. IIRC we had two tanks with capacity of around 16,000 gallons each, plus three trucks (2 5,000 gallon and 1 3,000 gallon)
The self-serve place would get a load of jet MAYBE once a month. Av even less frequently. To me, that is not enough turnover for Jet... I wouldn't go there.