After calculation my net salary is 6100 PLN
It's like I said before y'know, you won't like like a king. It's not a crap wage, but it's nothing special either.
If I was you I'd want more.
Like your rent is going to be 1000-1500 per month.
Food shopping 800-1000 per month
Internet & phone 200-300 per month
Travel (if you use public transport) I think in Warsaw it's 150 for a bus & tram ticket....I might be wrong
Electricity & Heating 200-300
That's your basic stuff
If you enjoy eating out, going to cinema, going to concerts/football matches then you're going to have to add all that in
September weather is usually decent enough, doesn't really get cold until January and thanks to climate change we've having shorter and shorter winters :)
Good luck with your decision