The surprising answer is “a long time ago.” Via The Economist:
In other words the decline in paid-newspaper circulation during the thirty years from 1900 to 1930 is 1/3 larger than the decline in the subsequent 80 years, which includes the alleged newspaper-killing internet phase. In fact, the largest 10-year decline was 1900 to 1910. That 10-year period saw a decline twice as large as the period since 2000. The internet may be ringing the death knell, but newspapers have been dying for a long time.