It is very, very common for piano lessons to be weekly and for a duration of 30 minutes. It is not just piano lessons, all other instrumental lessons I have heard of last for 30 minutes , once a week. At least this is the common experience here in Australia. The only exceptions I have heard of is for very advanced students who some have 45 minute lesson.
The reason why this is the standard is because it works well. 30 minutes is a good time frame to see what a student has practiced in the last week and make any corrections or adjustments that need to be made. There is also time to get started with new content, whether it is a new piece of music, or a new section in a piece of music, or a new focus (such as expression) in an existing piece of music. The thirty minute time frame allows enough time for questions to be answered. There is usually a few minutes to devote to scales, or sight reading or aural work. I must admit that I think thirty minutes is the perfect length of time for a piano lesson.
Not many people argue about lessons being thirty minutes long, but I have heard some people, not all, try to argue that a lesson every two weeks would be sufficient for a student. These people that have argued this point, are not instrument teachers or have ever had a music lesson in their life. I’ve even heard that some people think that the lessons are weekly, so the piano teacher can make more money. This is simply not true. If a piano teacher wanted to make more money then they would get more students and considering people are having piano lessons through an online platform like Zoom, now more than ever, piano teachers can find more piano students.
The reason why lessons are usually weekly is because of one word – momentum. If piano lessons are every two weeks, then any chance of momentum with learning would be lost. With weekly piano lessons, a person can’t really put off practicing because it is not a long time between lessons. Also a week gives a person a chance to get some sort of traction with their pieces of music, but it is not long enough to have mistakes ingrained and therefore harder to correct. I remember when I was a teenager still taking piano lessons, there was a period of a couple of months where my lessons were once every two weeks and even though I don’t remember that much about it, I do remember the feeling that it did not work for me.
If you are looking at taking piano lessons from a person, not an app, then remember that they are an experienced professional and they know what is best with regards to timing of lessons. Trust them. They are giving you the best information.