If you're not sure where (in what menu) an action in Rule Machine action can be found, there's a list at the end of the Rule 4.0 docs called "List of Actions," and it can be helpful in that regard: Rule-4.0 - Hubitat Documentation
For your actual rule, someone asked a very similar question in another thread and got more or less the same rule in the end: Turn Light off after 30 minutes. Might have pointed them here had I seen this first. 
(This seems to becoming a common enough request that I wonder if it could be squeezed into a stock app like Simple Lighting/Simple Automation Rules. I don't see a way to delay the "off" of a switch with that app right now--looks immediate to me--though I did see this alluded to in another thread as a possible solution. I'm not sure it is.)
