When did the average family have a microwave in the home?

The Bureau of Labor Statistics does keep data on consumer buying habits, and does have data on microwave ovens.

By 1986, 25 percent of households owned a microwave. Assuming the rate of growth stayed the same, by 2000 50 percent of households would have owned a microwave. It depends on what you define as average, but the threshold was probably reached sometime in the last 1990s.

This would vary by country, In the US, the rate of adoption once cheaper models became available in the late 1970s was rapid: "Figure 1 shows that as recently as 1979
less than ten percent of U,S. households owned
microwave ovens. By the late 1980s about eight
out of ten homes had microwave ovens." -from http://ageconsearch.umn.edu/bitstream/27501/1/22030045.pdf To my eye, Figure 1 in this paper show the 50% ownership mark in the US to first be surpassed in 1986.

In Great Britain, the year that more than 50% of families owned an item was used to furnish the family in the TV series "Electric Dreams" with that item- similar to the US, they did not get a microwave oven until the mid-1980s: http://www.open.edu/openlearn/history-the-arts/history/history-science-technology-and-medicine/history-technology/electric-dreams-technology-makeover