Last year I was curious as to how often the term "heavenly mother," appeared on the official Church site, so I did a search. I wasn't very impressed with the results. There were a total of 11 over the past 40 years.
Most of the references are from the 70s and 80s. The one reference in 2000 is from a manual that was originally written in 1979, but was revised in 2000, so I don't think that point is really indicative of what has been published by the Church in recent years. In general, Heavenly Mother was referenced "a lot" in the 70s and 80s and not at all in the past 15 years.
After fretting about this, it came to me to go back and search "heavenly parents." So I did.
Because some of the references were to the same article, I counted each article only once. This means that if there was an article in the Ensign that was also published in the Liahona, I counted it only once. There are peaks at 1995, 2000, and 2002. The Proclamation on the Family came out in 1995 and the 2000 and 2002 peaks are the result of the Church re-publishing manuals in 2000 and 2002 with the Proclamation printed in them.
In this graph, we can see "heavenly parents" was referenced regularly in the 70s and 80s and early 90s. In the years following 1995, many of the instances of "heavenly parents" are quotes from the Proclamation. Despite this, I think it's safe to say that the average member, from Primary age and up, is hearing "heavenly parents" more than s/he would have in the past.
I hope that this increase in reference to "heavenly parents" will continue! I'm interested in hearing your thoughts about these graphs.