I would have said forty years. This is much the same style as the movie and TV parodies from Mad circa 1969. And that's kind of the point, really: the creators of this strip are being faithful to the traditional style of the book, and succeed admirably at this goal.
The folks who've complained here just baffle me. If you went to see a show called An Evening With Groucho Marx would you storm out and demand your money back because the lead performer didn't sound enough like Patton Oswalt?