Like any human, I understand teacher's needs, but as a student when I have booked a 60-minute lesson I expect a 60-minute lesson. It is completely OK for me when the lesson continues 58 minutes, I can accept if sometimes the teacher will ask for a 55-minute lesson. Any delay up to 5 minutes is completely OK for me, but a longer delay creates problems for me because I have a very busy schedule.
I have mostly never booked 30-minute lessons, because it is too short, and to have 25-minute lesson has no sense at all.
I don't like when teachers offer 45 or 50-minute lessons in their profile because it is difficult to compare prices with other teachers. Lessons of these teachers look cheaper when they do not. If the whole platform works with shorter lessons we should accept it, but in another case, it is a little bit cheating.
Teachers, who need a rest can have a window between lessons. I understand that many teachers do not do it because students sometimes cancel lessons.
I have never seen inside an italki teachers interface, and never will.
But I believe rightly or wrongly that teachers can mark in their calendars their available times. So that all they have to do is to mark every two or three hours as unavailable; for 30 mins or an hour. Let me know if it is correct or not?.
I suspect your problem is to do with English lessons; due to the vast overwhelming number of people seeking cheap or free English lessons.
As a student:
I understand that teachers are humans who need breaks etc. Still, when I book 60 minutes then I expect 60 minutes. It is no problem when the 60-minute lesson has a tag on it: 50 minutes, 55minutes or whatever. Then I know and take this into account. But a teacher (like anyone else) who is more than very exceptionally late, is no option for long term serious matters. I know that in some cultures this is not seen as a problem... but for me and many of my Japanese, Chinese, American, Swiss, German, and Scandinavian students it is.
As a teacher:
First of all, I try not to do anything that would annoy me as a student. Being as much a student as a teacher, I am well aware of the many little things which could matter... So, when I offer a 60 minutes lesson, it is 60 minutes. Being late is a no-go, so I start on the second (ask any of my students). When there is another lesson directly after this, I also finish on the second. To be able to do this without being rude or abrupt, I indicate my lessons as 58 minutes, at least I used to. Now, I usually have breaks between lessons, so I can be flexible at the end. Actually I don't care too much for the time, because each and every lesson is fun for me.
IMHO, any lesson length shorter than 45 minutes is just a waste of time and energy. Focussing on the learning process, ideal online lesson length is anything between 60 and 120 minutes. This is the limit for students. A teacher can go much farther.
From my point of view, time slots of 15 minutes would be perfect in italki. So, a regular lesson would be 4 units, and lesson length could vary: 45, 60, 75, 90 ... minutes. This would also open the possibility to have 15 minute breaks. Now I have to choose between non-stop and 30 minute breaks which are too long and too short at the same time...