You are billed per hour, for each hour since the market in T7 Cloud Simulation instance becomes available. Start of the exchange can take approximately 10 minutes, but you are never billed for the time it takes for the instance to start up, until the exchange is available for your use.
You start the T7 Cloud Simulation instances for yourself only; the instances are then billed from the moment the market becomes available, regardless of whether you connect to it or not (its similar to a parking pricing scheme.) There is also an automatic termination feature, which stops your T7 Cloud Simulation instance when its maximum run time has elapsed, in case you forget to terminate it manually. This is by default set to 4 hours (meaning that by default you won't be billed for more than 4 hours), but can be adjusted at any point from instance start up until its termination within the interface. Instances can run for anywhere between 1 and 24 hours.