Well it depends on what you were doing. Clearly you were listening to music the entire time. Apple quotes up to 30 hours of music play on the 3gs. So assuming you had the iphone on sleep during the whole 10 hr 35 min then you would have used up at best around 35%-40%.
Considering how that is under perfect optimal circumstances, and you were most likely using a few other features on the phone, such as texting or other apps, as well as changing songs etc, I'd say that is pretty decent usage.