Clarification on EC2 pricing

Hi all,

I'd like to use an instance with lots of vCPUs to benchmark a specific test I'm running locally.
Let's assume I chose a compute instance, like c5.9xlarge at 1.818$/hour (or even more vCPUs like c5.24xlarge).

If I create the instance, launch it, download and install some software (like gcc) and use it for one hour, and stop it, I will only pay 1.818 USD ?
Do I need to pay if the instance is down ? If not, how long will it be kept ?

Thanks for your help.