Believe it or not, the jailbreak has INCREASED my battery life. Let me explain. Prior to the jailbreak, I was finding that the phone was losing 5-10% per hour even while sleeping. After the jailbreak, this was resolved. I then noticed that Skype was no longer multi-tasking. After the jailbreak, Skype for some reason has reverted to an on/off app. In my opinion, Skype was the cause of my battery drain, and the way my phone is functioning in it's jailbroken state is actually preferable to the way it was when Skype was sucking my battery dry.
Just my experience. Ymmv...