Is that unusual? My previous phone - an SE P900 - would normally use around 10% a day on standby, and would last a well over a week if fully charged at the start.

My current phone, an SE P1i, claims about 18 days on standby, but it is too new for me to verify that. (I'm seeing a lot less, but I think the battery will take a few charges to mature, I'm playing with it more because it is new, and I suspect the backlight sometimes comes on accidentally in my pocket.)