If a parent came to me when I was a soccer coach and asked "how long will it take for them to learn how to play soccer?" I would be absolutely dumbfounded. I couldn't answer that. 

How about school? If a parent asked "how long until my child learns math?" A math teacher couldn't answer that question with a simple response. Do you mean 2+2 = 4 or the law of sines and cosines? Big difference.

If I were to sign my kid up for four 30 minute piano lessons and then take a break, what should my expectations be?  Not sure they would even master "Mary had a Little Lamb."

As adults and parents why do we think kids can learn a complex athletic skill in just a few 30 minute sessions. Swimming is more than just getting your face wet and getting to a target. Learning to swim is a process that is full of building blocks, milestones and even plateaus. It is not just swim, float, swim.