The majority of frozen feet show up right away, like the ears do. However there are a percentage that show up at weaning and some more that show after feeding for a while.
I was buying calves one year and missed a sore foot on one steer, wasn't bad in October when I bought him, but I barely made 1000 pounds by the time he was pretty bad on that foot. I got a slot at the packer and he just graded with about a pound to spare on the light side, not much fat cover, but enough I got out of him without a loss.
A frozen foot will sometimes show up after the next cold winter. The foot is more suseptible to frost after it has had some freezing on it. The calf could seem 100% sound untill the following January in the feedlot then his foot refreezes and he comes up lame.
That is the reason buyers can discount the froze ears, but they do take advantage of it a little much.