In the original production the chorus was on stage only when you hear them on the album plus the recording engineer. It isn't all that much time although the numbers may have been cut for time. However long they're on the stage would be a director's decision, but it isn't in the writing.
The script was published in a hardcover edition and could probably be found on a used book site like Abebooks.com.
In that chorus was Debbie Shapiro who evolved into Debbie Gravitte and who covered Lucie Arnaz. One of the bartenders heard an understudy rehearsal and told me (I was across the street at another theatre) that if Debbie ever went on, she would blow Lucie off the stage. And this wasn't bitchery, Lucie was liked; it was a case of big talent. This was about 30 years ago.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher
