I've been carrying the 3DS with me to work for the last 3 days and I've been getting an average of 5 miis per days, with two of them being the same all 3 days. One day I got 8 people though, yay!.
I also get 2 or 3 during my walk to work and back home, but it's only 15 minutes and I live near the business area so it's not like there are tons of kids around... There are two schools near my house though, so maybe if I leave home a bit earlier or a bit later, I might be able to hit a bunch.
(ps. I live in Fukuoka, not as many people with 3DS as in Osaka, where I would get at least 10 or so every time I took the subway, did some shoppings and then went back home)