He mentioned he wanted the Routine to be a “manual run” Routine (i.e. a Scene). You don’t have the “Turn off after” option for manual run Routines for some odd reason. Your second suggestion is the way to go.
Another way to do it would be to use the Edge Counter Utility from @taustin to turn on a counter when the AC is turned on. Then have a second routine to check for the counter value and when it reaches the desired amount of time, turn off the AC and the counter. The counter can also be turned off manually which would negate the second Routine from executing.