When I started studying music my teacher said the secret to practice is doing a little bit every day. It doesn’t matter if it’s only 5-10 minutes because the regularity is better than one or two longer practice sessions per week.
He was right. It’s about developing momentum and making sure the last thing you did is at the surface of your thoughts and easily accessible. The skills we don't practice atrophy over time. How long is too long? It's a matter of degrees but every bit counts when you're trying to improve.
You might have heard that doing something 21 days consecutively forms a habit. Maybe, but it doesn’t mean the your body will go on autopilot. On day 22 and every day thereafter you still need to make the decision: am I going to maintain my momentum?
Regular practice leads to mastery. There is no other way.