I've seen it up to 8 bars often, just depending on how long I hold the regen on. I've seen it WAY below 4 bars. Hell, I've had it down to 2 bars, and maybe even one, I can't remember. It didn't stay down there long, but going on a blitz from 55 to 100 will definitely put you down there =) I've only had ONE instance where the assist turned off, and that was climbing a hill on the way to Vegas.