It gets very complicated, because a lot of it was orchestrated by Mel Karmazin, who then ran the combined business, and merged it with Viacom. Somewhere in the midst of that was when the era ended.
As I understand it, and this is highly condensed:
In his early days, Mel was a sales rep for CBS. Eventually, he founded and was head of Infinity Broadcasting. In 1992, Infinity, the owner of the legendary WBCN in Boston, bought rival WZLX.
In 1995, Westinghouse acquired CBS. Westinghouse took the CBS name as its own, and spun off its electrical power and military units, among others. Infinity was still a totally separate entity.
Shortly thereafter, CBS acquired Infinity, a move that was orchestrated by Mel. As a result, Mel was made head of CBS Radio, which was renamed Infinity Broadcasting, a division of CBS Corp. Around 1998, American Radio Systems, owner of WRKO, was acquired by CBS. Its properties were folded into Infinity Broadcasting, except for certain properties which were put into a trust, WRKO among them. WRKO was soon acquired by Entercom.
Mel was on track to become head of the entire CBS Corp. However, Mel decided - or perhaps it was decided for him - to move on, and the Infinity Broadcasting brand was immediately discontinued; the radio unit was once again CBS Radio.
Until today, 17 November 2017, that is.