I always feel a bit melancholic when I read all this wise advice on "you must practice at least one hour a day" and so on. Once upon a time it was a motivation issue for me too. Nowadays I want so badly to practice but I CANNOT because my life does not allow it. I practice whenever I have the time but so very often I just don't have the possibilities. Don't talk about "priorities" here because noone would disagree about my priorities if they had known the full story ... well, those who don't like piano playing would of course say that I should drop piano practice completely but I refuse to do that.
But it is right now an utopia to have a regular practice schedule. And the nearest teacher lives so far away that I must reserve at least half a day to get there, have my lesson and go back home, so this "30 minutes a week" is also out of the question. The most intense schedule I have had as an adult, was one hour every fortnight but it often had to be adjusted to suit my job schedule. I run my own freelance business and I need to adapt to very short lead times and deadlines all the time, I can seldom plan for the next week.
On the other hand, I had 20 minutes every week when I was a school kid and it was a worthless schedule if you ask me. To little time to work in-depth with anything, and too little time inbetween for practice - after all, I had school too. And I lost my motivation that way, so I did not practice as much as I could have, so I had to show up, week after week and make apologies to my teacher ... You know, in this way your original goal (to learn to play the piano because you love it) somehow gets replaced with "please your teacher" and that takes the fun out of it. And I was too young to be able to analyze the situation. Today, the problem is the opposite, now I struggle to find practice time, not to get out of it ...
I believe it is good to have lessons often when you are a beginner, need a lot of guidance and have short assignments. Or when you are advanced and have hours to practice every day. But I am advanced in the aspect that I work with long and difficult works, full sonatas and so on. With my irregular practice possibilities they take many months, sometimes years, to learn. There is absolutely no point for me in having short lessons where we can work only with snippets of a piece.
Having said that, my conclusion is that lesson time and frequency is a highly individual matter, it is no solution that is ideal for everyone. I have reached the stage, though, when I do not pity those who cannot motivate themselve to practice even though they have the possibilities - either you love to play the piano, and then you play the piano as much as you can (and that varies according to your circumstances), or you are perfectly happy with the situation just as it is and that is ok too. The choice is yours. But the drive to practice (or not to) should not come from your teacher.
Otherwise, it is good to have a teacher. Playing the piano can be quite lonely and you need feedback and input from someone who both is paid to give you honest feedback and input and also is skilled enough to do it properly. The lessons are good for structuring. You can, if you are lucky, get help to participate in group gathering and recitals and that is incredibly fun. My own piano teacher is also a very sweet and nice person who I look forward to meet ... unfortunately she is on sick leave, long-term, so at the moment I don't have a teacher to see.
I have an offer to get lessons online from the other side of the planet but this requires arrangements that are not feasible at the moment.