Duration takes the time from the first person in the Space to the last person in the Space. Billable time is only counted when two or more people are in a Space simultaneously.
Usage is calculated per second at any time where there are two or more connections to a Space. If there are two connections to a space, and one drops out and rejoins, the time where there was 1 connection will not be billed for.
This also means that your teachers can join a Space to prepare for a lesson for as long as they like and you won't be billed for the time they spend alone in a Space.
For more Pricing FAQ visit our Pricing FAQ page.