depends how hard you press and whether you're on a flat, uphill or downhill road. Most say about a mile or two (unless someone wishes to correct me)
In stealth during the summer (without A/C), you could easily go 2 miles. I've went 3 miles under ideal conditions starting from the top of blue (6 bars) with no traffic on an almost completely flat road at 30-35 MPH.
With 6 bars, a flat road and steady 25 MPH I was just over 3.5 miles. You have to slowly accelerate to speed. One quick press of the accelerator and it could cost you 1 bar, that is if the ICE doesn't kick in.
