"Eight eights." But because it's slightly unusual and easily misunderstood, I think you need to rely on creative description. Personally I would say something like "it is just the numeral 8 repeated 8 times: 8, 8, 8, 8, 8, 8, 8, 8." Or write it out on a scrap of paper. "Eight times eight" is bad, because with that word order it sounds like multiplication, 8 x 8 = 64. They probably would not type in "64," but they might say "huh? What do you mean?" "Eight eights" is good. "Eight, eight times" is OK, because that's not how we express multiplication. "Eight, repeated eight times" is better.

Definitely not "eight times eight." That's a math problem, not a code. You could say "eight eights" or "a series of eight eights." If there were more variety in the numbers (which would definitely make it a stronger password), the most natural thing would be to say the value of each digit in group of two, three or four: two three, four five, six seven, eight nine two three four five, six seven eight nine But if every digit is the same, it would make more sense to just tell them how many times to enter the digit.

Natural would be things like: The password is: ‘Eight eights in a row’ or ‘Eight characters, all eights’

many thx for your detailed explanation.