It's going to vary depending on an individuals head:ear:bushyness ratio.
Mine personally usually sit just below that nobbly bit, but when my hair gets longer and bushier, I let them go down about half way of my earlobe.
Good trick to get rid of the bushyness is to get a razer with one of those "precision trimming" blades, and lightly glide the precision blade over your sideburns. Go easy though, you don't want to have long hair then whack of your whole sideburn.