How much time is required to maintain a certain level of proficiency in a skill? For example, if I practice learning a language for one hour a day for a year, if I switch to once a week or once a month will I be able to maintain the level of proficiency I have attained? How much practice would be required to avoid atrophy? Do other skills (e.g. card magic, picking locks, juggling, martial arts, musical instrument) follow a similar pattern, or do they require different amounts of maintenance? What is the difference between skills that are masterable (e.g. riding a bike) and require no practice vs perishable skills?