As of July 1, 2013, all Gliffy licensing (including trial and evaluation licenses) are processed through Atlassian and your my.atlassian.com account.
For server add-ons, you can extend your add-on trial up to 5 times - in other words, for up to six months. Extend your trial by generating a new evaluation license key from the Atlassian Marketplace. Click Try it free and you'll be directed to generate a new license. Paste this license key into the add-on listing in your UPM from your Atlassian host application, and you're all set.
For cloud add-ons, you cannot extend your free evaluation period. All cloud add-ons are immediately subscribed by a user, and we provide a free evaluation period. This is a minimum of 30 days and ends on the second billing cycle after you first subscribe to the add-on.