The ‘it” – motor learning improvement- has many influencing factors (physical, emotional and psycho-social, practice design, genetics, etc)  
Forming motor memorys  (creating new neuromotor pathways) can occur in minutes or months.
See the PantherTec Glossary for definition of terms such as neuromotor pathways, muscle memory, long term potentiation, perception action coupling, affordances, constraints and other cool stuff.