From the "unusually fair billing policy" section on the pricing page: > Our plans are limited by how many readers you have, and how often they visit your publi

From the "unusually fair billing policy" section on the pricing page: > Our plans are limited by how many readers you have, and how often they visit your publication. We only enforce these limits on a 3 month rolling average, so we'll never make you upgrade plans if you happen to have one big traffic spike or particularly successful month.