Depends on whether they are having you start drinking the contrast at home. Usually they have you drink two bottles of contrast and one glass of water w/an hour between each.
I pick the contrast up and drink the first bottle at home 2 hours before the scan is scheduled, I drink the second one just as I leave the house and they hand me the water to drink when I walk in the door. By the time they take me to the back and have me change into a gown, etc. my gut is ready for the scan and we begin. The scan itself takes maybe 15-20 minutes. By the time I'm dressed and ready to leave they have the CD of the images ready for me. And I'm out the door.
The first CT scan we did they had me come in to drink the bottles of contrast and water there. I'm NOT good about
sitting around waiting for some procedure so I had a good talk w/them the next time I was scheduled for a CT scan and we worked out the above arrangement.
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.