For example, at the moment, naked female breasts are absolutely not prohibited by law in the city of New York and its residents can easily, if they have such a desire, walk with naked mammary glands.
When women cease to be regarded by society as property in respect of which sex characteristics can be taboo, and this gender itself is a belittling factor. What are open breasts? It's "indecent, cover up!" or "Oooh, boobs, let me feel, perhaps you're ready for anything!" So when such reactions become socially unacceptable - that's when.
By the way, the "wild" tribes of Africa and Oceania do not make such a problem out of the female breast as the "enlightened" carriers of European culture. And somehow nothing died out from overexcitation.
Yes, it could have been "legalized" for forty years - if only a little "squeezed" in the sixties-seventies.
In general, social progress (and naked female breasts are, in my opinion, exclusively progressive phenomenon) is not a linear process. Both revolutionary and reactionary trends constantly coexist - and under normal conditions they approximately balance each other. But from time to time, some additional factors are superimposed on the picture and breakthroughs happen, when the "social paradigm" changes quite radically (then usually there is some rollback, but, anyway, the world is already different).
So if it happens some new powerful surge of social and cultural activity (as in the United States and then in Europe in the 60s), then the breast issue will be "resolved" in a couple of years.
In addition to all of the above:
Depends on how religions will spread over the next years (decades and beyond). Christianity, Judaism and Islam discourage women from exposing their breasts. If people are more "going" to religion, then this is unlikely to become the norm.
Just let women and girls begin to strip her publicly for granted and that's it. Well, of course, simultaneously declaring this “it's just a breast, that's okay!”.
The process will go on, in a dozen years and the culture will change.
When female breasts evolve to very small, or disappear altogether as a class. From 10,000 to 1,000,000 years.
When genetic engineering erases the difference between the sexes for versatility. For example, as in "Knights of Sidonia". From 100 to 10,000 years.
When cosmopolitan nudists unite the world into a single state where dissent will be suppressed. From 30 years old.
In the event of a nuclear or other global catastrophe, when humanity is technologically thrown back. From 30-50 years after the disaster.
In case of victory of radical feminists, the establishment of matriarchy, and the development of parthenogenesis in them. Never.
In case men also grow breast (Only 2 conditions - equality). From 10,000 years.
If all people on earth go blind, and no one will care. From 30 years old.
If you live among Aboriginal people, where is it. From 1 minute.
If you are on the internet. From 1 minute.
In case we lose the seven-hour war to an alien race, which will suppress sexuality with the help of radiation, thereby controlling the population. About 5-10 years after the accident while exploring portals to other dimensions.