Is there any way I can check how long motion has been active? I need to do something like, "if there is somebody in the room for at least 15 minutes, then do something(change HVAC temperature as per this room)"
Several ways to do this in RM...
Trigger on motion detected
Wait for 15 min
Check to see if motion is still active
then off to the races!
You'd probably also have to set your motion sensor to a 15 min motion clear time to prevent the rule from constantly retriggering...
You can use Time Since Event condition to test a particular duration. Or you can use Duration in a Wait for Conditions. Or you can do time arithmetic on %time% if the device was the triggering device.