The manual for the original Nintendo Entertainment System said "Do not operate continuously for more than 15 minutes."
Sometimes legal departments force them to include those recommendations in order to err on the side of "absurdly overcautious." And sometimes the recommendations are merely prudent! I'm not saying you can wait longer, just that installers may have a motivation to exaggerate.
But unlike an NES system, if your septic tank develops a problem you now have liquid and solid waste flooding your yard and home, and your toilets, sinks, and showers are useless until it's fixed.
I feel like the failure mode for electronics is "randomly catches on fire" which can be just as devastating, if not moreso. Nobody has ever died because their toilet backed up. Their house burning down in the middle of the night is a different story.
Sometimes legal departments force them to include those recommendations in order to err on the side of "absurdly overcautious." And sometimes the recommendations are merely prudent! I'm not saying you can wait longer, just that installers may have a motivation to exaggerate.