It's most likely irrelevant as you would be unlikely to use your laptop as a power source; but the iPhone can be charged via a USB connection. Thus if your laptop battery is charged up you could just plug the phone in for an hour and there you go.
Rate of power discharge seems to vary dramatically depending on what you are doing. For me if I use wifi a lot then I need to recharge daily; if I just use it as a phone then it will last 3-4 days. I think the ipod function uses a fair amount of power too, but I don't use that bit enough to make a judgement.