How long has illiteracy been happening?

Answer 1

Since the first dictionary was written; prior to that there was nothing to refer to for spelling and meaning. Illiteracy has always existed by present standards, if you don't grandfather in the lack of reference.

On a side note, it appears to be rising since the advent of the internet and the improper reliance upon spell checking features; there is no way a spell check feature will recognize every word as the correct word for the context