Jim, I'll start by saying I'm no expert on reedie engines but there are a variables to run times.  RPM, prop, fuel, compression, all effect run times.  Nitro content will have a big effect and changing one or more of the above will change run times.

No, 8cc, full out screaming will probably get you 2-3 minutes, depending on the prop and compression as Jim said. Even less if you have a bored venturi and a dual bypass cylinder. More if you throttle it. But that 8cc built on tank will suck air in your loops and overheads way before you're finished. When I do stunts with those, I do all of them early in the flight. But I'm just a sport flyer so it doesn't matter what order I do them in.

I believe Cox made the tanks for the sport flyer who was not worried about competition.   The competition engines were the TeeDee and Medallions.