I imagine this is different for each store and location.
The biggest consideration is that your store has enough stock, so if there are several Apple Stores in your town, make sure it's a decent-sized one.
I went to the second biggest Apple store in my town and the lines were a bit shorter than the flagship store and i was able to buy it in a reasonable amount of time. The smallest stores in town were out of stock much quicker, but these were tiny stores.
I didn't care that I was the first or 10th person to get the iPhone or iPad and didn't want to wake up early, so I just went 30 minutes before the store opened at 9:30am. I had to wait about 2 1/2 to 3 hours as about 150-250 people were in front of me in the two times I bought iOS devices on opening day.
If I had to do it again, maybe I would get there one hour before the store opened. The Apple staff will give you a number while you are waiting in line so you will be guaranteed to get an iPad.
It is though, far easier to just pre-oder the iPad as soon as it becomes available online. DON"T WAIT, if you do this as the initial batch will be out of stock soon and you'll have to wait for post-launch shipping dates if you don't pre-order right away. I waffled on the first day that the iPad was available online and missed the launch date shipping date, so that is why I went to the store so I didn't have to wait a couple weeks for it.