You might have already seen Clay Shirky's now famous speech about cognitive surplus given at Web2Expo and the dramatic comparison of the time spent watching TV (200B hours a year in the US) and building Wikipedia (100M hours total).

He mentioned another number I thought was interesting: "In the US, we spend 100 million hours a weekend watching just the ads" (fast-forward to 5:52, or read the transcript). The order of magnitude seems right but I can't figure out how he arrived at his estimate. Here are the inputs I'm working with.

- Number of hours spent by men watching TV on weekends: 6.98hrs (less for women, but I'm keeping the math simple). I don't know if the number is an average across the entire population or only accounts for those who watch TV.
- Number of ad minutes per hour of TV programming: 16 (wiki), which means 3.72 hours of ads total for a two-day weekend (16*6.98/60*2).

Now, to arrive at the 100M hours number, we need to assume an active audience of 26,881,720 (100M/3.72hrs) viewers on each of the two days. How accurate is this number? I'll try to check with our media folks next week, but drop a comment if you have ideas.

Of course, this little calculation assumes that Shirky's remark was not a mere rhetorical device (hey, if we just stopped watching ads we could build a wikipedia in a weekend) and that people do watch all of the ads throughout the entire 7-hour TV binge instead of doing laundry or zoning out.