Your mileage may vary, but with a mild motor, you can go a LONG time with a 4800. I know for a fact that you can do at least 15 minutes in an on-road car under racing conditions without hurting them. IMO, that's a lot of continuous track time...your eyes will probably be blurry by then...LOL
I wouldn't worry about a cutoff unless you were running a full-on mod. If you're running something mild, just charge it after every quarter hour and you're good.
After a while, you'll be able to judge how close you are to dumping it by how many mAh you need to put back in.