20+ hours might work for some but to be honest "muscle memory" (if you believe in it) doesn't require that necessarily.
Practice at the edge of your ability can really hone your action and make your limited time pay off. This is deliberately hitting big hooks, big slices, fades, draws, high, low...hitting 4 different clubs to the same flag...and so on. I enjoy The Inner Game of Golf by Tim Gallwey for great ideas on how the brain and body can work together to maximize your time.