Woo -

It's probably somewhere between our two estimates. Standard car battery is rated at 45 Ampere Hours, which comes out to about 2MJ - You don't need all of the battery charge to actually start the car, and chances are, the battery is not completely dead. My estimate of 8 days is to bring a battery with no charge up to full capacity, which is plenty of juice for several starts.

I think that while starting the car, the battery has to supply 100 - 200A, for a time of a few seconds - if we say 200A for 5 seconds then the actual power used to start the car would be would be 12kJ - if the dynohub generates 3J/sec, then that is 4,000 seconds or about an hour to generate the power needed to start the car.