Some analog studios used clocks to calculate the final bill for their clients. Others were paid flat rates with amphetamines, cocaine and hallucinogens. The flat rate worked well because they couldn't see the clock clearly and would often lose track of time... staying up for days at a time, then sleeping the entire weekend, waking up soaked in booze and vomit Monday afternoon.
Some unusual versions of popular songs resulted in these sessions. It was a golden age.
Yes, they used clocks. The band would come into the studio and the engineer would look at the clock and start charging $75 an hour and the band would do cocain and argue about what song to do first.