I launched an advertisement on FAcebook and i got 18 messages from people who wanted to learn Dutch. I was impressed by the amount of people who wanted to learn Dutch, and I wonder why that is.
Poles are really keen on learning languages in general, but what you'll find is that most of them will be poor students who won't have the means to pay for a teacher.
Note that this was just to check, and there would be no charge (like i offer dutch , you offer me polish).
Doesn't really mean anything then. You should put an advertisement offering classes and you'll see the response then.
So i guess there should be a decent amount of interest of people who want to get private lessons, like indeed as you said, around 4 to 5 hours a week, maybe at a rate of 60 zloty/hour
60zl/hour is unlikely to happen in the current environment with someone who has no experience as a teacher and no knowledge of Dutch philology. 40zl/hour would be more realistic, and even then, it's going to be unreliable.
In a month that would be a decent amount.
Not really. Polish people are somewhat unreliable and they won't hesitate to cancel at short notice or simply stop showing up. Relying on private lessons for income is - usually - a bad idea.