I'm using "48V" (14s 20Ah medium-c-rate LiPo-like pack, 58.3V fully charged) and a 40A 12FET, and a plain 9C 2807 in a 26" wheel, on the heavy 150lbs+ CrazyBike2, and i can eventualy get to 35MPH (EDIT: make that 32MPH; I misremembered the data)
if I have a long enough off-road stretch (since max on-road allowed is 20MPH around here). (it will reach 40MPH unloaded off-ground wheel speed).
A higher-amp controller would let me reach those speeds faster on my bike (300lbs+ including rider), but on a lighter bike or a lighter rider or both, the 40A controller would likely be sufficient to accelerate to the faster speeds relatively quickly.
The biggest limiting factor is the battery itself: with a low-c-rate 60V (70V+) NiMH pack, it sagged so much that it only reached 31.5MPH max. I forget what the 48V (59V) LiFePO4 pack would let me get up to, but I don't think it was much beyond that, if even that fast, also because of heavy sagging.
You could use an RC LiPo pack, 10-20Ah, to emulate what I am using now.
As for how long these speeds can be sustained, well, I don't have a long enough run available to be able to test that.