No brainer: weekly.
No, this is not a suggestion meant to line your teacher's pockets.
The primary reason bi-weekly lessons won't work is because you won't practice thoughtfully and regularly if you don't have a "call to accounting" (lesson).
This means your progress will be painfully slow and you'll be immensely frustrated. Leaving a day or two between practice sessions (because you know you won't be having a lesson "this week") means that 80% of the gains you made the last practice session will be lost. Therein lie the seeds of your frustration.
Meanwhile, what you "see" is that (1) you're too dumb to play piano; (2) your fingers are "too stiff/uncoordinated/etc."; (3) you're too old because you can't seem to remember what you did last time you practiced; (4) your teacher isn't very good.
If you don't have the time or money to take a lesson every week, wait until you do. You'll be much happier with your piano study.
You might be interested in amount of time to practice daily.
copyright 2002, Martha Beth Lewis, Ph.D.
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