fear-peile:
I'm not an expert on Warsaw rental property and can't answer your question, however it sounds reasonable providing it's a gross income.
all:
please note, that in IRE it's "bedrooms" that are advertised while in PL it's "rooms" - this include both living- and bed-rooms therefore a "2-pokojowe" (2-pok, dwa pokoje) means livingroom + 1 bedroom.
It is also worth mentioning that 2 - 3 room (1-2 bedroom) properties are most "liquid" on the Polish market - easy to let, easy to sale. I wouldn't personally go for anything larger than 3-room (2-bed) investment apartment.