On broadcast TV - a typical 30-minute program will only be 18-20 minutes long, with 4-6 commercial breaks totaling 10-12 minutes. There is no way that the time between commercials will be 15-minutes in this case.
Now - some channels that show movies will make adjustments ... they will go longer between commercials at the beginning of the movie to get you hooked - and then have commercials every 3-4 minutes towards the end knowing that if you've invested that much time to watch thus far you're not about to leave before seeing the end.
On earth - I would use a timer, a clock, or my watch to judge the passage of time ... not the unpredictable whim of a broadcaster! Although, if you're watching MASH on TBS ... from the start of the theme song, which goes directly into the first round of commercials, to the start of the show is about 3 minutes.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain