I have my 3DS in my coat when I walk to work, to get play coins and catch any potential street passes on the way in. I'll usually leave it sitting in my coat while I work, so I can take it back out for more at lunch and again at the end of the day.
However it's not clear whether or not I'm using up a lot of battery this way, especially as it's left sitting in my pocket for hours while I just work. Does anyone have any information on how power hungry standby/streetpass are?