Breastfeeding question

How long does your baby take to empty one breast? I heard one recommendation to have them spend 20 minutes on each breast, but sometimes I feel like that's not enough and wonder if he's getting the fattier hind milk if I switch him to the other side too soon. Is it better to let him nurse longer on one side and less on the other -- and start with the opposite next time, or better to nurse evenly?