Well, honestly, that kind of depends. First, will you be taking any classes or seminars? Will you be looking just at the quilts in the show, or will you want to shop the vendor area too? Second, how much standing and walking can you do? How many breaks will you need?
I've been to Paducah once and Houston 4 times. I have always stayed the full length of the show. The thing about these huge shows is you can get both physically and visually overwhelmed. I found that after a couple of hours, everything started looking the same, my eyes had glazed over, and I needed to sit and take a break. What I learned to do is spend a couple of hours in the show, take a half hour break, spend a couple hours in the vendor area, take an hour break, then go back to the show. If there was something I found that I liked in the vendor section, I would write down the booth #, what the item was, and the price, and come back later to purchase it, and here's why: Often times I'd chew through my budget and find something I would have rather purchased after I was out of money. Other times I'd find the same product somewhere else for a little less. Sometimes I'd go back to purchase what was on my list and wonder what I'd been thinking.
I can also recommend that you take a back pack, a bottle of water, and some snacks.