I think the real answer is "it depends".
You should consider: How fast, how many hills, how much stop & go, how hard will you be peddling and how quickly do you want to accelerate.
My usage is about 6-8Ah from a 48V A123 pack during my 20 mile commute to work. My total elevation change is about 300ft, avg speed is 20MPH, and I peddle hard on an upright Mtn bike with commuter tires. YMMV (Your mileage may vary).
Eric S.