We aim to have the absolute best uptime possible. We don't have regularly scheduled interruptions or customer-affecting maintenance. And planned interruptions are kept to an absolute minimum (we have not had any planned downtime in the last year).
We publish our public uptime via the third-party service Pingdom. It's available here: http://stats.pingdom.com/p7x42pb3mmgt/480614
We also have a status site (http://status.chargify.io/), independent from our primary systems, that helps to keep you up-to-date on any issues we may be experiencing. You can even subscribe to get notifications on changes in our application status.