Whats an appropriate number of billable hours for an event lasting 18 hours of actual "show" time? What's a proper ratio of planning time?

Obviously you can't go into any demonstration / proceeding of any kind that you're paying a (usually degreed licensed) white colored professional for (accountant, architect, lawyer, surgeon, etc) without preparing in order to give that client the best of your abilities. Even non-license professions like sales, marketing, acting, software developer need planning time for a job. Same with blue collar professions like plumbing electrician framer etc need to assess a job come up with a plan of attack.

But what's inappropriate ratio of planning time to showtime FOR WHITE COLOR CAREERS?

Whats an appropriate number of billable hours for an event lasting 18 hours of actual "show" time? What's a proper ratio of planning time?
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