However, in the article, Norvig himself points out the worthiness of learning several languages, and doesn't claim that you have to get on a ten year treadmill for any individual one, or even a subset. My impression was that Norvig was really targeting the people for whom whatever-language-in-a-fortnight-and-a-half would be their first exposure to programming at all.
As you gain more languages, provided you are judicious in choosing, so shall your learning of each of those languages be more rapid than the prior.
Does a month per language seem a bit rapid? Yeah, it kind of does, depending on the time spent in study and tooling. The pragmatic programmer guys suggest learning a new language every year; that seems wholly attainable. In reality, the author studied a language to what he considered a sufficient level in a quarter. That also doesn't seem that unreasonable, depending on the level of mastery desired. Could be that both perspectives have more than a grain of truth.