4% an hour, I'm getting over two days from my battery..
It depends what you're doing with it, obviously. I'm keeping 3G turned on in a weak signal area, checking for stock updates every 5 minutes, weather updates every 15, internet time sync on, Location service on, push email enabled for an exchange server and for a hotmail account, and checking for emails on an ISP account every 5 minutes. That lot uses up 4% of battery per hour in stand-by.
If you're not doing that much, obviously your battery will last longer.
If I turn all of that lot off so the data connection isn't being used at all, turn off 3G, sever the data connection, and have it do nothing at all except listen out for phone calls and text messages, then it'll go down only 4% in 9 or 10 hours.
Conversely, before I did my last hard reset, I twice saw it go from 100% to zero in less than 9 hours while in stand-by all the time.
I still haven't quite worked out what it was I installed that did that! Task manager and 1% battery icons, maybe....