Hello Miriam! Lets just be honest here, 100+ hours a month is absolutely insane! That makes learning a language a full time job! I am a Spanish learner and take Italki classes every week. I also take classes at my school. At school I have 3 classes a week at 1hour and 30minutes each. This is a great amount but also very extensive which can be overwhelming. Especially, when it comes to homework and processing new grammar words and rules. On Italki I take either 1 class or 2 a week which is perfect for me! Each class is 1 hour. So in all honesty it really depends on you and your personal learning style. Some things to take into consideration are...

- How much free time do I have to dedicate to lessons?

- Will I have time to complete the course work for that class (if any) by my next lesson? 

- What is my purpose for learning? If you are going to Costa Rica next month then take more lessons, if its just for fun then take less

- Do I have other practice times at places such as school/university, work, home life, etc?

Please consider all these things to decide your perfect amount of classes. If you need any more help on anything feel free to contact me:) 

Thanks for all the comments! If I wouldn't work, I could imagine taking more classes, but on the other hand, then I might not be able to afford them. Even with only one class per week, I have trouble doing exercises in between and reviewing what I have learnt. I definitely have to work on my time management...

@Aliph

Wow, 30 to 40 classes per week plus homework is really a heavy workload... I do learn several languages at the same time but not very intensively (and not by taking classes), so I think it's still ok.