I was curious last night; I had just returned from a trip during which I ran a turn-by-turn nav app and had my 4S (iOS 5.01) plugged into a car charger so it was almost fully charged at the end of the day. Rather than leaving it in the charging dock overnight I put it in only until it showed 100% and then took it out - at 11:15PM.
Now I did have both WiFi and Bluetooth turned on, and two email accounts that push and several others set to fetch, and they in fact did that overnight - I had my expected emails in the morning, so this wasn't total standby - the phone did have some work that it did.
This morning at 6:05AM (6.83 hours later) it showed 96% - for a standby discharge rate of .585% per hour, which projects to something like 170 hours of standby. This is short of the 200 hour claim, but not hugely so; overall I'm pretty happy.