Just for comparison a military fast jet will fly 1500' on downwind. Spacing about 9000' from the runway centreline. Descent is commenced at the base turn point. In nil wind the BTP is abeam where the finals glide slope will intercept 500'. Speed on downwind anywhere from 160-250kts. This varies subtly with from type to type but is basically as I've outlined. Circuits take about 4-6 minutes each which is the same as oval circuits in CT4's, and PC-9's which fly a closer circuit spacing. Much more efficient than a square circuit. Extending base is a no no at military training airfields as is stuffs up the spacing of all following traffic.
Not saying that any previous posters are wrong just giving some info on how military circuits are flown.
Cheers