I'm an ambassador and I have exactly 71 streetpasses as of today. I average getting one every 7 to 10 days or so and I take mine with me everywhere. Don't expect to get a lot, but you will get some. My problem is that I haven't gotten a new puzzle piece (other than the Fire Emblem giveaway) in months!
If you live in the city you'll get at least one a day. However, there are days I've gone out and didn't get one even though I saw the person playing their 3DS.
Sometimes some jerks turn their wireless off because they honestly believe they'll be playing their 3DS the entire time it's on, in public until it dies without charging it. Which is both dumb and annoying.
...Life is tedious if it ain't flowing... Trump x E4's Caitlin 3DSXL FC: 4640-0379-8455 PSN (PS3+Vita): TehTrumpCard