You can do timed instances, and/or make the instances have timed job to shutdown after a fixed time (which is what I use to shut down an instance which only gets spooled up for occasional CI jobs after an hour).
+1. When I had to use AWS for batch workloads, which at the time at least didn't have a TTL attribute on VMs, I made sure that the VM first scheduled a shutdown in like 30 min if the test was supposed to only run in 10 min.