Good luck!!! The place is an absolute mad house on sunday! Not just walmart the entire complex, the later in the day you go the crazier it gets and you'll have to wait many light cycles just to get out of the lot.
Don't get your hopes too high
I got there at 2, no line at all for pickup. First item (4 pack steel folding chairs) was brought out right away, other item (crib mattress) was on the truck. If I haven't done any other shopping, it would've been a half hour total. Problem was I saw they were bringing the mattress just as I was going to check out for my regular shopping, so I went straight there figuring they'd ring up my items. They didn't, so I spent an extra 15 minutes to check out for the items I bought in store.
All in all it was 45 minutes in and out, and my wife took up that exact time up at Sears after dropping me off, before picking me up. Not terrible to be honest.