It would be much better if you used a "timestamp" instead of a set_task for counting down the time.
Assign a variable (I'll use X in this example) X to get_systime()+(However many seconds you want the room to be rented for), and whenever someone tries to rent/use a rented room, it checks to see if the rent time is up. If the rent time is up, it won't let the previous renter use the room, or it will let another user rent the room from that point on. It's better to compare seconds than to use a set_task.