Fan for 15 minutes or until I tell it to stop

I actually just posted what I'm doing in a different thread. The commands might be a little different as I have an ecobee.

But I have 3 virtual switches, one for 1, 2, and 3 hours. All accessable from Alexa. I also have installed a app called one at a time, which allows only one of those 3 virtual switches to be on. But besides that, it does what you are looking for, and then some.