There are three ways to do this now depending on exactly what you need. As is typical, smart things is very powerful, but not very intuitive.
One) if you just want the light to always turn off after it has been on for 15 minutes, you can just use the power allowance feature in the official smart lighting smart app. But in that case no matter how you turn it on it will turn itself off again after 15 minutes. And that’s not what most people want.
Two) if you want to sometimes have the light turned on on a timer, and other times turn it on and have it stay on, you can use a virtual timer there is a how to article with the instructions for this in the community – created wiki.
This method not very straightforward nor flexible. Samsung could EASILY add an option in Routines that says “In x minutes: 1. turn off 2. Resume most recent Routine, or 3. Run a different Routine”. It’s very simple and straightforward. A SmartApp could do this too if anyone is willing to create it. The programming is a bit over my head.
It remains a mystery as to why SmartThings has never offered a simple rules engine. People have been asking for this from the beginning. There have been several community – developed alternatives, but it remains a missing piece in the official features.