I remembered reading in Jeff's 48 Hours with Nintendo 3DS article that he had some concerns about Sleep Mode and battery life, specifically that if you left the 3DS on in Sleep Mode that there wouldn't be much of it.
Or you know, what he actually said, that people probably want more battery life rather than more Miis.
WELL GUESS WHAT MOTHER FUCKERS.
So I decided to leave my 3DS on all day, in Sleep Mode, from 9 AM to 3 AM. Yes, that is my day.
That's about 18 hours of non-gaming goodness, except I played like 30 minutes of Chrono Trigger but whatever, and the final result was "Oh I guess it still has a full charge."
Now, yeah, it probably wasn't full, but it said it was full. The blue bar was on it and it went all the way from the back of the battery symbol to the front of the battery symbol, which is to say that there was, indeed, much battery life left.
So yeah, leave fucking Street Pass on, it doesn't matter.
Also, I don't know about you but I think the battery life is pretty good. I've been playing a shit load of Chrono Trigger and I've barley lost any power at all, never more than half. As for 3DS games, aka Street Fighter, it loses battery power more quickly, though if you are only playing in short bursts I'd suspect that 3 - 5 hours is more than enough if you are, you know, playing it like a portable, and not playing it for 3 hours, nonstop, at a time.
That's not to say it's excusable, but you know, I'm fine with it so far.