@Gaveston, I oddly couldn't find any info on why 8 shows is the standard. I'm sure someone must know. Thanks for trying, Nate. I am to blame here, as I'm sure I knew the history once upon a time and have forgotten it. If I can find an answer, I'll let you know.
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FOR EVERYONE ELSE: I was by no means arguing that actors should do more than 8 shows per week. The shows in Vegas that do 12 or more shows per week are very different in form.
And, yes, the best actor/singer/dancers I've known have given their all during those 8 shows, so I am by no means arguing that performers have it easy.
On the other hand, in my last theater job, I worked 70+ hours/7 days per week for 9 months of the year. (40 hours/week the other 3 months.) And I was a marketing writer.
That was pretty exhausting, too.