When did microwave ovens become very common in North American homes?

In 1965, Amana, a subsidiary of Raytheon, produced the first popular household microwave oven. The microwave oven really took off though in the early 70s.

"Tappan Stove company of Mansfield, Ohio bough a license from Raytheon in 1952 and tried to sell a large, 220 volt, wall unit as a home microwave oven in 1955 for even lower price of $1,295 but it was too early and this oven did not sell well. Raytheon bought Amana, manufacturer of household appliances, in 1965 and they made the first popular home model, the countertop “Radarange”, at a price of $495. Litton Industries, developed new configuration of the microwave in the ‘60s. It was short, wide shape that we all know now and it used special magnetron feed which allowed for oven to survive a no-load condition when it had no food to absorb the waves. By the 1970s prices fell rapidly and microwave ovens became common in many households."

As for the earliest (clearly not mass-market) commercially available microwave, this is pretty interesting:

The first commercially available microwave oven was, Raytheon built, "Radarange". It was made in 1947 and was very large. Each made was 1.8 meters tall, had 340 kilograms weight and was sold for $5,000 which is today somewhere around $52,000. It also had power of 3 kilowatts and was cooled with water. One of the first “Radarange” ovens was installed in the galley of the nuclear-powered passenger/cargo ship NS Savannah.

http://www.historyofmicrowave.com/microwave-history/microwave-oven-history/

I'll bet McGruff's mom didn't have one in 1965; mine sure didn't. In 1973 I worked in a cabinet shop that did upscale kitchens, and they were just beginning to include microwaves.

http://microwavemasterchef.com/microwave-oven-history/

Sales really took of in USA and Japan from the ten’s of thousands in 1970 to the millions by 1975. By the late 1970’s, technology improvements and cheaper electronic parts meant that microwave ovens became more practical and affordable. In 1986, 1 in 4 US households owned a microwave oven. By 1997, The U.S. Bureau of Labor Statistics reported that a staggering 9 out of 10 American households owned a microwave oven

McGruff's mom did not have a microwave in 1967. McGruff suggested to his sisters that they get a microwave for their mom for Christmas one year, c. 1987. McGruff's mom isn't much for new technology and this microwave hasn't seen all that much use, but she still has it and it looks and works like new. McGruff doesn't give his mom a cheap gift.

In 2000 when he originally answered this question, McGruff had a microwave, but he didn't have all of the internet he has available to him today, and although he had a number of rudimentary search engines, some of them quite good but no match to Google, those search engines couldn't find what wasn't there.

McGruff would say that 1976 is probably a pretty good answer to this question. McGruff had graduated from high school, and a year or two later was beginning to establish his credit. He opened his first charge account at Montgomery Wards with the purchase of... you guessed it, a microwave oven. No more cold pizza for breakfast for McGruff. If a 20-21 year old was buying a microwave c. 1975-76, they were probably very common by that time. It was a monstrosity, though. McGruff parted with it in his divorce, c. 1983.