Last August we (just 2 of us) walked up and were seated at lunch in the time it took my boyfriend to go to the bathroom and come back. They told us 20 mins. There were a lot of people in the waiting area though. If you are just 2, you will probably have good luck but I have no idea what a bigger party would of had to wait.
I think it just depends. When we were there last Wednesday, they were telling people not to wait in the line to check in if they didn't have a reservation because they weren't taking any walk ups at all.