From documentation:
“Heroku usage is computed from wall-clock time, not CPU time. This means that usage accumulates over time as long as dynos are enabled, regardless of traffic or activity.”
https://devcenter.heroku.com/articles/usage-and-billing#computing-usage
750 hrs is enough to cover any single dyno for an entire month. Essentially “non-production” apps are free in that regard. (unless you have another non-web process running, like celery etc)