When did Indycar meets become like F1?

Can someone help jog my memory? Cept for Indy, American oval races have traditionally always been one-day meets, hot laps in the morning, qualifying at noon and the races in the afternoon. Sprint car meets are usually still that way (well, mostly evening and nite), but Indycars have at some point evolved into three-day meets, like European Grand Prix. When did that transition happen for the first time? 70s, 80s?

Probably the first time a promoter figured out he could charge the suckers extra to watch practice.