Streetpass relay points only store data from one system at a time for each game.
Let's assume you and John only have Mii Plaza games in common. John tags the relay first, and it saves his data, but doesn't send him anything. Then you tag the relay and get John's data. If you tag the relay the next day and John has tagged it again in the meantime, you'll get John's latest tag, but if you were the last one to tag the relay then you won't get anything.
Now let's say you and I both have game XYZ (with Streetpass functionality) and John doesn't. You tag the relay first and get nothing, but the relay stores your data for Mii Plaza and XYZ. Then John tags the relay and gets your data for Mii Plaza. Then, if I tag the relay, I'll get your data for XYZ and John's data for Mii Plaza.
The data does not expire (as far as Nintendo has disclosed). So if you tag the relay today, no one tags it for an entire year, and then John tags it after that, he should still get your data.
I hope that helps.
Also, here's Nintendo's explanation: http://www.nintendo.com/3ds/nintendozone?relay=on