Three years, his first 3 years to be exact, he then removed it, and it stayed that way till he retired. It was also the year he really broke out so to speak, when he didn't wear his helmet, he tallied something like 53 goals, and around 120 points.
Lafleur wore a helmet the first three years of his NHL career. Ironically the first season he took it off he recorded his first fifty goal season and his first 100 point season. That was in the 1974/75 season.