I’m tired, so someone else will have to help you with the details, but see the following post (this is a clickable link).
There are many different ways to do it. If you always want the light to go off after the same amount of time, just use the power allowance feature in smart lighting.
If you want it to sometimes go off after 15 minutes but at other times stay on for longer, you’ll need to use one of the more complex options in the post.
So as with many use cases, smart things is very powerful, there are multiple ways to solve the same problem, but none of the solutions are intuitive.