Instances don't have a minimum runtime, they have an added runtime of 15 minutes. Or to get even more technical, you are billed for an added 15 minute runtime

Instances don't have a minimum runtime, they have an added runtime of 15 minutes. Or to get even more technical, you are billed for an added 15 minute runtime but the instance can still be shut down. However if an instance is needed within these 15 minutes and the last one was shut down, then a new instance is created and billing-wise it will act as if the old one was running all the time. An example to illustrate: T+00m request comes in, instance #1 is created T+05m request comes in T+07m instance #1 gets killed due to google spilling coffee on the server T+10m request comes in, instance #2 is created T+25m instance #2 has been idle for 15min, gets killed - Total charge: 25 minutes