2500 Pln a month
You do know that you will lose 19% of this to tax a month and to be honest 5000zl per month is not that much of a big deal, and your quality of life will be okay but not fantastic.
For example if you are going to be here long term, how are you going to settle down and build a house on declarable income of 2500zl per month? (as the other 2500zl from your private lessons is cash unless you declare it)
Please I am not trying to put you off, I just think you need to be a bit more realistic here :)
I have been here eight years now and slowly moved away from teaching (although I still do a little bit) as it is not the most profitable career if you want to settle in Poland long term.