For a new one?
Probably about negative $2,000 per month.
It takes several years for a portrait studio to even get to the point where they can get up to zero. Here is a good rule of thumb for an average studio
Year 1: Lose $25,000
Year 2: Lose $15,000
Year 3: Break even
Year 4: Make $15,000
Year 5: Make $30,000 (quit day job)
Year 6: Make $60:000 (spouse quits day job)
Year 7: Make $75,000
Year 8: Make $100,000 (hire employee for $30,000) Net $70,000
Year 9: Make $140,000 (Hire second employee) Net $80,000
Of course those numbers are estimates, but I want you to understand that almost all photographers lose money the first several years they are in business.
That is actually a good thing. Photographers who actually make money the first year, are usually not spending enough to build their business infrastructure and lack long term goals.