First, frequency is the more the better. Especially during the earliest phases of each focal point. A famous quote by a classical musician states,"If I miss practice for 1 day I notice. If I miss for 2 days my wife notices. If I miss for 3 days the world notices." And that is after a long career at the top of his world.
Second, if every movement of every practice is done at the level that requires 100% focus, a 20 min session will be useful.
Also, when you are not lucky enough to do that, I have noticed that mindful practice makes the stroke come back very quickly. I, due to a terrible ability to schedule, regularly spend months out of the water and I am lucky if I can get in 2x per week regularly. Every time, the first time back feels awful. The second feels good and the third is back on track.