I don't know about kids cartoons, but I just watched the original Star Trek episodes from the late 60's and they are 50 minutes each (for a 1 hour time slot) so that's 10 minutes of commercials. Hour long episodes are now about 40 to 41 minutes in length, so the number of commercials has doubled since then.

Its gonna differ per country. I remember in the 90s 3 commercial breaks in a movie here in NL was default. I was shocked and annoyed when I learned in early 00s it was every 20 min. I don't know what it is now, I avoid commercial TV like the plague. Though public broadcasting also still has them. As a kid and young adult I used to be able to sometimes laugh at commercials. Nowadays, some commercials appeal more to me due to age diff, but many don't because they use youth language. On top of that I find practically all commercials are condescending, stupid, and annoying. Their attempts to deceive and manipulate are laughable, ineffective, a waste of my time, and a waste of my mood. Nothing good comes out of me watching commercials. Not for the advertiser, nor for me. I cannot remember the last time it stimulated me to buy something I otherwise wouldn't. That is, from TV commercials and ads on the Internet. Product placement, embedded marketing, marketing buzz works fine. I suspect there's so many TV commercials because it is ineffective. If less people watch than before, its more expensive to buy the rights, so they gotta advertise more. Or perhaps its corporate greed.

SciFi channel once aired the original uncut TOS episodes. Each episode took an hour and fifteen minute time slot. Shows how much commercials have increased since than...

Taking a look specifically at the Big Bang Theory, up until Season 10, most episodes were around 21 minutes. Season 10 had episodes around 20-21 minutes, and the current season (11) hovers around the 19-20 minute mark.

When I was in High School, our English teacher actually wrote and produced a TV show and was telling us about her experience and some of the things she had to do to cut episodes in the right amount of time for commercials. At the time (mid 2000s), the average was 23 minutes of show per 30 minute episode.

Looking across my library at recent shows, it seems to average about 21 minutes nowadays.