for me it seems to be loze to an Hour anx a balf from half a battery. Of course i try to never expire an ipod battery. Or now an iPhone battery. I have a almost 4 year old blk/wht screen click wheel 30gigger that is still jamming like a champ. So i wouldn't know about charging from empty. It takes the same for me as the ipod does. No difference with the iPhone.
Obviously when using the internet and what not during charging it is going to take longer. The iPhone charges to 80% or so rapidly and then trickle charges until full. So it does take a little longer then expected. Mine takes about 3-4 hours to get to the trickle charge state.