While the listed prices for Heroku products are given as monthly costs, we prorate all costs to the second. You can scale down your dynos when they're not in use.
$7 per dyno/month is the maximum you're charged if you run a Hobby or Basic dyno 24 hours a day, 7 days a week, since your last billing cycle.
For example, a Hobby or Basic dyno costs $7/month. Running 1 Hobby or Basic dyno costs roughly $0.25/day. If you use a Hobby or Basic dyno for a Review App and scale the dyno to zero (or delete the Review App) in less than a day, it would cost you less than $0.25.
See Usage and Billing for more info.
For info on how to optimize the performance of your dynos, see Optimizing Dyno Usage.