It depends on the number of bars of the battery indicator. I left mine in standby in the evening with 2 bars and next day the batteries have been discharged.
I always leave the device on standby. I read somewhere else (I think the develeper mailing list, but not sure...) that some apps won't let the cpu go into standby even with the cover on. So, I guess it's best to close all the applications if you plan to let the device on standby.
With couple of days old battery it didnt even consume 1 bar when it was most of the day on standby. First bar was consumed after 2nd day (i had used the 770 for like 2-3 hours for offline-html-document reading + calender/to-do-list.)
For those who dont have the device yet, it has 3 bars of battery.
edit: it was offline in the night because i thought it would use the battery without any reason (shutting down takes few seconds and starting it in the morning maybe like 30-60 seconds).
