I don't think there ever needs to be one. I'm a great believer in that the body adapts to whatever you force it to.
I wasted years "undertraining" because everyone said "you can't possibly squat 3x a week and not burn out" or "you shouldn't squat and deadlift the same day" etc.
Sure, if you're used to only squatting once a week, upping it to four times a week might be a struggle straight away, but it's not something that can't be built up to. Christ, Eddie Izzard ran 43 marathons in 51 days with no prior significant training, do you think he'd have managed that if he listened to what people thought he was capable of or what was possible?
I hate imposing arbitrary limits on people. If you can't handle squatting more than once a week then I think you need to get fitter for the task. Not decide that it's impossible.