I am going to be there from 2pm to 9:50 pm on a Wednesday.
Toilet aside, I am guessing that's a good time to take a cab to the city and return by 8:15pm or so? I will look at the other threads for transit suggestions for that time frame.
If you've got 7 hours, exit the airport and walk/taxi to the Le Meridien/Al Bustan Rotana hotels across the road. You don't have to have a reservation to use their loos
You could also then get a decent meal at the 20 or so restaurants.