"Capable of being read by the average viewer" means just that... the time an "average" person would take to read the notice. The average reader isn't super fast or super slow, so if you look at your notice and think how long it would comfortably take someone to read it, that's probably the right length.

On full-length clips or DVDs, we make this notice about 7-10 seconds long. On our 30 second clips, we make this about 1.5 seconds long, which is a little short, but under the argument that it isn't reasonable to have the 2257 notice be 1/3 the length of the clip.

I saw an HBO special on porn a while back on pay-per-view, and their 2257 notice was literally a flash on the screen. I had to go back about 3 times to actually see it, it was maybe 1/3 second at most. It also didn't name an individual custodian. That notice was definitely not compliant, but I doubt we'll see Time-Warner get busted for 2257.