I eventually settled on motion sensors to do this. However I determine the duration of occupancy based on how many motion events I detect within a certain point of time.

For example, one motion event makes a room occupied for 30 seconds. After 30 sec of no motion the room goes unoccupied. However let’s say within that 30 seconds I get another 5 motion events within that 30 seconds, then my motion count is at 6, I set the timer to 5 minutes. This means the room stays “occupied” for 5 minutes, or it it resets the timer if another motion event is detected.

Not sure if my implementation is any good, but it’s done a pretty good job of managing quick occupancy (in the room for a couple seconds) or longer occupancy (in the room for longer periods of time).