Dyno Hours for Hobby in Heroku
With a Free Dyno you are entitled to 550 hours, if you add a credit card you get an extra 450 hours (total 1000). They go to sleep after 30 min idle time.
Hobby Dynos are not free but they do not sleep (always up and running) so you do need to worry about the hours, you will never pay more than $7 (this as Aug 2020).
Hobby Dynos are still prorated to the second so you can spend less if the Dyno is stopped/paused (scaled down to 0).