Italki is a marketplace, not an app. You learn by buying live lessons from human teachers. Each teacher works individually. Italki doesn't supervise the content or design of the lessons, they aren't standardized. Each teacher decides what kinds of lessons to offer, how long they should be, and how much to charge. You look at each teacher's individual profile to see what they offer. 

Many teachers offer lessons of 30, 45, and 60 minutes. When I take lessons, I like a length of 45 minutes if the teacher offers it. Each teacher decides how much to charge for the lesson.

Italki keeps teachers happy by scheduling lessons, collecting the money from the student in advance so that the teacher is reasonably sure of being paid, handling payments. Italki keeps students happy by not paying the teacher until the student says the lesson is complete and satisfactory, and by listening to student (and teacher) complaints.

How well you learn depends on you and the teacher. I am learning Spanish. All three of the teachers I have used have been excellent: skilled teachers, and completely professional. Each has had their individual style. I am at an intermediate level so I have plenty of room for improvement.

I don't know if Italki teachers can help you. When you look for a teacher, you should state that you are already at a high level and looking for someone who can help people at an advanced level. Most teachers offer very low-cost trial lessons. The risk of trying out a teacher isn't very high. 

My experience with italki as a way to buy paid lessons has been excellent. The no-cost features--the notebook, "answers," and "discussions" sections--are a nice extra, but unimportant, and are not a system for learning a language. The facility for meeting language companions, people who simply agree to meet via Skype, is OK, not great.