I think it was Gorn,who in another post opined that there was a point of "diminishing" returns.
I subscribe to that idea in the context that you can reach a point in over-practising where your mind simply becomes bored and tired and doesn't even focus on the matter at hand any more.
Muscle memory doesn't take as long to acquire as most people think it does.
The important thing is that what you're learning registers in your memory.
Muscle memory is actually brain memory. Muscles don't remember anything.
If you don't play bass for a year, then you become "rusty". If muscles remembered things , you wouldn't be "rusty".
Just my experience, but lots of times if I'm having trouble learning something I do best to just drop it, come back the next day and I can then play it. It makes sense from a neuroscience perspective in that the down time allows the nerve connections in the brain to form what the OP is calling "muscle memory". The six hours a day thing is like cramming for a test - not very effective in really learning something.