Usage calculations are in one hour increments
Note that Frame uses hours in increments of one hour. Booting a Sandbox or Production instance will consume one hour of usage, even if you shut it down after just a few minutes. Also, anytime you publish - you will incur one hour of usage for each of the instances in your production "pool" -whose size is determined by your "maximum" capacity setting.

Also note that min and buffer instances incur hourly usage whether users are connected or not.