Change effective on 11 December 2017
All apps using legacy free dynos have been migrated to use account-based free dyno hours.
Free dyno hours are distributed across all of your apps that run on Free dynos. This allows you to run a single app at all times, or run multiple apps on a more occasional basis.
Each Heroku account receives a monthly quota of Free dyno hours. When any Free dyno is active (i.e., when the app is not sleeping), it consumes hours from the quota. As long as there are dyno hours left in your monthly quota, your Free dynos can run.
Learn more in the legacy free dyno migration FAQ.