When did "learnings" become an accepted countable noun?

I share with you the jocular nuance of the word. However, a quick Google NGrams search shows that some time in the mid 1920's it started to become very popular very quickly and has been slowly reducing in frequency since a peak in the 1950's.

A perusal of the hits shows that it's been used in the same manner, things that have been learned, all these years.

It has always sounded 'off' to me, like you said uneducated or like a goofy neologism that would never really catch on. And yet, it caught on a while ago.

Although the Ngram chart in Mitch's answer provides a heartening view of the decline of learnings in published texts included in the Google Books database (from the bad old days of the 1950s through about 2008), a more up-to-date look at the Ngram chart for learnings (starting, as Mitch's chart does, in 1910 but continuing through 2019) shows that the word has been on the upswing since about 1990 and that it has really begun to pick up steam in the past ten years:

This may reflect, in part, the vogue that learnings has enjoyed in business-speak in recent years, as MBAs—the same people who gave us "If you have to pose the ask, you can't afford the spend"—have embraced the notion that teachings beget learnings. I spend a lot of time editing business articles, and I can tell you that the enthusiasm for learnings in that sector shows no sign of abating. We're still in the early stages of a second wave at this point, but a wave it appears to be.