If you are using a recent version of Ubuntu Desktop, you can configure this pretty quickly using dconf-editor. Here's how:

0. Install dconf-editor if it is not already present:

   
   

   sudo apt update
   
   sudo apt install dconf-editor
   
   

   
   



1. Open dconf-editor:

   
   

   

   
   



2. Click the magnifying glass to start a search, and type "sleep-inactive":

   
   

   

   
   



3. Edit the sleep-inactive-ac-timeout value to 1800 (30 Minutes):

   
   

   

   
   



4. Edit the sleep-inactive-ac-type value to shutdown (or something else you might prefer):

   
   

   

   
   



5. Be sure to save your changes.

   
   

That's all there is to it.