I just bought a new book ("Mastering machine applique" 2nd edition, by Harriet Hargrave) and in it I found the following statement:
"A study at Johns Hopkins University found that it takes six hours to permanently store any new physical skill into your memory. If the storage process is interrupted by learning another new skill, the first lesson may be erased. The study showed that time itself is a very powerful component of learning; it is not enough to simply practice something, you have to allow time for the brain to encode the new skill. "
I equate this to what a lot of people refer to as "muscle memory". I did a quick think about how long it took me to "get" FMQ stipling, and if I think about the hours of JUST stipling, it's about right.
I didn't double check the reference to Johns Hopkins, but I have no reason to doubt it.
Thought it was interesting enough to pass on.