It was done in the Final match of a major tournament held in Los Angeles in 1971 or 1972. Fred Whelan produced the event for several years in a row and all the top players came out to play in it. I was there!
From Lou's wikipedia page:
His nickname, "Machine Gun Lou", derives from his stunning the crowd and fellow competitors by running 150-and-out in straight pool in 21 minutes against Allen Hopkins in 1973.[1]
In 1973, he won his first World Championship. Lou earned his famous nickname that same year when, in an exhibition with Allen Hopkins, he ran 150 straight balls in just 21 minutes.