I keep it at level 22 because I like a BRIGHT, WHITE screen all the time. That is the whole idea of PaperWHITE!
Anything less than 22 changes the white to really off color.
Battery time is irrelevant. I did not buy the PW to have a great battery. I'll wait for the new 3D batteries to be in the Kindles this Christmas.
Anyway, getting 10 to 12 days of battery life at level 22 light level is just fine for me.