Depending on the register, and depending on how well-organized the cans were, it could have gone incredibly quickly.
When I worked at K-mart, the customers who knew the routine would keep the various flavors in separate stacks. I'd count the number of cans in each stack, type in that in the keypad, press Qty, scan one can, and bag the stack. Other stores require you to press Enter after each can. Buy a 24-pk, and the cashier has to press Enter 23 times.
Most customers will just bring a crapload of cans though, with several different UPCs. Well people, if you don't want the inventory system totally screwed up, we need to scan every unique UPC. Otherwise your poor Fluffy might have to endure Beef and Liver when it's his Seafood Deluxe night.
I think I've seen it recommended that you NOT vary your pet's diet, or it can make them picky, especially if you go "Aww, don't you want this? Here, I'll give you this instead," and open another can. Just give them the same thing every day. They might be fussy the first day, but to a cat or dog, finely-ground animal mush is still damn tasty, no matter what it happened to be previously.
My cats have been eating the same flavor of catfood for most of their lives now. Every day they get it, it's like it's the greatest thing in the world all over again.
What I'm trying to say is, if the customer and cashier are both smart about it, and the register has a working Quantity function, you can quickly scan and bag 100 cans of cat food as 10 items. 10 stacks of 10, sorted by UPC. It works. I've done it.