Running Cydia runs my battery down. Playing games runs my battery down. Talking on the phone for a long time runs my battery down.
I have WinterBoard and run some mobile substrates and other tweaks. My battery goes down to about 95% after a day of standby only. If I use it for some calls, some texts, some Cydia, some music, it may go down to about 70%. If I do a lot of Cydia and a lot of SHSHing, it might go down to 50%. I've never run the battery down in a day. I charge every night, but I have no issue with that.
If you have some tweaks on your phone, it shouldn't really affect battery power more than 5-10%. If you need that percentage, then run a stock phone or add no tweaks. For me, I hate how the stock phone looks, so I change it.
Agreed. My personal experience with an iPhone 4 running first 4.0 and now 4.2.1 has been that a lot of this "jailbreaking/Winterboard will kill your battery" is nonsense. I run a phone with 133 packages, according to PkgBackup, which many people here will tell you is insane. The phone is neither bogged down nor does it lose battery any faster than my husband's virgin, absolutely stock firmware. I've run side by side comparisons to check. We both charge every night as well--and always have, even when I was running stock. My phone has been restored exactly once, when I went to upgrade to 4.2.1. As Saurik has recommended, I reboot it every few days or so. And I routinely kill backgrounded apps.
What does kill my battery?
Bluetooth is THE single biggest culprit on my phone. Every time I start to think I've got a battery problem, I look up and realize I've left BT on again.
The rest--
Brightness above 45% or so (I'm typically indoors, so keeping it lowered isn't an issue, and I crank it up when I'm out driving, etc.)
Actual usage as a phone (heaven forbid we actually USE the damn thing as intended)
Games--even theoretically low impact ones such as Solitaire have animations
Surfing
iPod--though this is in part because I leave the screen on full time when I'm playing anything
Navigon (which, to be fair, is usually used with brightness up at 100%, the iPod backgrounded and streaming audio via Bluetooth while the phone wrestles with varying degrees of 3G signal strength. Do all that and you too can watch your battery drop 10% in 20 minutes! LOL)
Resprings
Note that only ONE of these things is JB related. Running a theme (or even multiple themes) has no noticeable effect my battery--but tweaking that theme six ways to Sunday while it's on the phone, respringing between changes, will drop my battery life in a given day considerably.
Especially for brand new jailbreakers, who are typically non stop playing with their phones--before you start pulling everything off your phone or restore it, reboot it and then try actually running it for a day of normal usage without changing a single thing on it. Maybe two or three days. Stay out of Cydia, don't change themes, don't respring more than once and only if you feel the phone is sluggish. I think most people trying that are going to find that at most, their battery difference is in that 5-10% range that dudeabiding mentions.