I'm going to say for at least a month in Warsaw. If I rent a flat using 'biuro' - who pay commission: an owner or a guest? How big it usually is?
If you rent an appartment using real estate agency, there's de facto standard fee - 1 month rent, which is splitted 50/50 between the owner and the tenant.
Because the owner will be charged too, it's unlikely somebody will be interesting in such short-term rental through the agency, so you should look for direct offers (pl: bezpośredni).