Actually, it'd be every 15 minutes to average a 7 minute wait.
And to find routes, simply look at Southwest's schedule between major airports. Or any commuter airline between New York and Chicago. If they're filling a 150 seat plane every 45 minutes, you could certainly fill a 20 seater in 15.
Slots at major airports are very expensive, and scheduled very carefully. You could only do this if you used minor airports, which would probably kill the convenience factor.
edit: Actually there probably are pairs of cities where it could be made to work by using small, low-traffic, convenient-to-the-city airports which can't handle large planes. But now your main problem is chicken-and-egg: until you have a lot of customers you can't possibly run a frequent service, and the customers won't be interested unless it's frequent. I suppose you could take off every twenty minutes regardless of passenger load, but in that case your operating costs are going to be enormous from day one.
Actually there probably are pairs of cities where it could be made to work by using small, low-traffic, convenient-to-the-city airports which can't handle large planes.
I think this was the intent -- fly out of Palwaukee, not O'Hare.
