I don't have cattle, but I have a ewe that has had an Allflex sheep ear tag in for almost 7 years now and, while the number has faded from black to gray, it is still legible. I liked that about the Allfex tags so I went ahead and ordered the sheep mini size for my flock.
It's not the eartags, it's the ink that's used to mark them. Get the Y-Tex ink. The marking pens are notorious for fading. Also, I used to use the Y-tex and Allflex tage, but have lately switched to the Z-tags. They are harder to apply, but they stay in much better.