Once a week is regular enough to keep up some good momentum whilst also making sure you’re not practicing the wrong things. It’s so much harder to break bad habits than it is to never learn them in the first place.
Fortnightly lessons are possible and are sometimes a financial necessity. They work best for students who are motivated enough to self learn, or are already independent musicians.
How Long Should A Guitar Lesson Be?
I generally recommend one hour lessons.
This allows enough time to appraise the prior weeks practice and improvements before introducing new ideas for the week ahead.
Children under 12 are the only students I’d recommend a half hour lesson for.