Whatever they're using on the backend to determine you've read too much for free and now have to pay is a bit wonky. It's also very unclear how often they reset that for the individual user. It's been reset at least twice for me, with no real obvious correlation to a specific length of time that I've noticed. It's probably not the approach I'd have taken, but Wired has been doing it for years (a site I coincidentally never visit any longer), and is still in business, so who knows.