Just a couple years, actually -- the first warnings appeared in 1966.
Which means, of course, the cigarette lobby had been doing its job for decades and suppressing public policy research. People in 1964, as today, knew smoking was dangerous. There were health professionals who, in the 1940s, opposed WWII cigarette rations, arguing they were not only dangerous to GI health but a cynical economic ploy to create hundreds of thousands of consumers. Gasp! Say it isn't so! Cap would probably give a righteous beat-down to those pencil-necked commie geeks.
Those lobbyists are in the pharm industry now . . . Take your meds, Tony!