Gaseous :
A small circuit I would hazard a guess at 2 - 3 minutes ? If so, any fast jet approaching would be between 14 and 20 miles away, and it's only in the last 30 - 45 seconds you'd have any realistic chance of spotting it. Plus, it tends to be those last 30 - 45 seconds are when you're busy making sure you don't screw up the approach and landing, pointing in one direction, so I'm not at all surprised you haven't seen them.
Depending on the sort of ops you do, have you found CANP any use ? Or even better, fly between 1630 and 0930 weekdays, and all day weekends !!