Range testing on my Walmart Mongoose bike with SLA battery (two 12v batteries rated at 12Ah each, in series = 24v, 450W motor) showed that when the battery was new, I could go (no pedaling, essentially no load except rider, flat terrain) about 14 miles at mostly about 16 mph with about another mile possible before going below 10 mph.
Looking at the batteryspace website, it looks to me like the equivalent battery is rated at 12Ah for a 20 hr discharge rate, and for 7.2 Ah for a 1 hr discharge rate. Meaning, of course, that the faster you drain the battery, not only does it run out of juice faster, but that the total juice it can deliver is less.
I'm not sure I see an identical specification for LiIon/poly batteries, but there is one spec regarding high rate discharge of being at least 90% of rated capacity.
Taking these figures, I would guess that a 4.2 Ah LiIon, around 24v, battery would last about 30 minutes at about 16mph.
Corroborating this calculation is a response by the moderator on the batteryspace bulletin board, which says that they would estimate an ebike with 600W motor and 37V LiIon 8Ah battery pack should go 12 miles. They say 450w motor 36v should go up to 20 miles.