HomePod settings to turn on and off a light

yes, you have to create a new automation, only you can decide when you want to shut it off.

Or may be you want to shut it off automatically 2 hours later, and in that case you can specify this in your initial switch on automation

In your automation choose “ Turn off”. Selections of times are limited, so if you don’t like any of them, create a new automation that shuts off.

Select "sunset" as the time. You'll need to have location services turned on.

Whenever you tell it to turn off. Set an automation for any time