I used to burn at higher rates, thinking it didn't make any difference. If you burn at too high a rate, sometimes you'll find a car player (or even a CD home player) that somehow can't recognize the disk.
Therefore, personally, I burn at 4x rate.
Which means however long the CD is, divide by four and there's your approximate time.
When the battery hits 10 percent, you run the risk of shutdown before a given disk is complete.
Paul