We've had some shirts made there, they turn out very nicely. I don't think you'll spend too much time deciding if you have an idea of what you want.
If you can think in advance of the characters, the color of shirts you'd like, whether you want the graphic on the front or back, and the phrase you want on the shirt (if any) it won't take too long to make your decision. You'll also want to decide if you want long or short sleeves, and the sizes you need. That will save a bit of time.
There is a time for creating the shirt, and I *think* it may be around 2 hours. Gives you some time to browse around DTD, maybe get a bite to eat. That probably varies depending on the number of orders being created ahead of you.
