I purchased SB-L320 batteries from MaximalPower. They're about $25 and run the back for an hour or so. I've never tested the actual length as I shoot more tethered than off the batteries. Anyhow, it's two years old and seems to give me about a 30 - 45 minutes at present. The SB-L160 batteries are about $15, but not enough cheaper to warrant buying them in my opinion.
http://store.edigitalmall.com/l320.htmlDave