I got 4.8 hrs like lady4u.
For every 5 mins the tub drains 24 gals of water. So in 10 mins it will drain 48 gals of water. Multiply that 10 by 6, to make it an hour, and multiply the 48 by 6 as well, you get 288 gals. Divide that by 60, and its 4.8.
Well I was wrong. That was for 288 gals. It would take about 28 mins. I made a table. !!