Found this on the web. It illustrates the % of a show's broadcast time that is devoted to commercials. These were all for main line shows aired during prime time hours:

1952 - 13 percent of the time was spent watching commercials (only 4 minutes out of every half hour!)

1958 - 13 percent
1964 - 18 percent
1977 - 18 percent
1994 - 24 percent
2004 - 30 percent
2006 - 30 percent
2007 - 30 percent
2008 - 32 percent
2009 - 30 percent
2010 - 32 percent
2011 - 31 percent