Change effective on 01 June 2016
Starting today, Heroku accounts have an account-based pool of free dyno hours for use on free apps. This replaces the 18 hours a day limit on free apps, allowing a free app to run 24/7 if needed.
New accounts receive 550 free dyno hours and you can verify your identity with a credit card for an additional 450 hours. Existing Heroku customers will receive a grandfathered amount of free dyno hours based on their usage in April, up to 2500 free hours (2950 if verified).