You want to find out how much of your Free Dyno hours quota you've used this month.
You can monitor your Free Dyno Hours quota and usage in your Heroku Dashboard or by using the Heroku CLI. Usage metrics are not live and may lag up to one day.
Using the Heroku Dashboard
Visit your Billing page: https://dashboard.heroku.com/account/billing
Using the Heroku CLI
Option 1: Show usage in hours
$ heroku ps -a app-name Free dyno hours quota remaining this month: 799h 43m (79%) Free dyno usage for this app: 200h 16m (20%)
Option 2: Show usage in seconds
$ heroku user:info . . . account_quota: 3600000 quota_used: 614439