At first with my Touch I set auto-sleep at 5 mn and OFF at 10 mn, but it was really short, so for my Glo, I set it at 10 mn for auto-sleep and OFF at 15 mn. That way is really good for me and though I read with light on when it's to dim to read without I'm about 50% battery life left (wifi off)
I charged it fully when I got it just a month ago today and then only connected it a few times to my computer to load books from Calibre. I've read 8 books (about 300 pages each) and began an other. I'm very pleased with it