@Rawalanche: yes, 10 hours are a bit too much, but I let render through the night, so maybe after 3 hours it was good. After 10 hours it reached pass 120. Is there a specific number of passes that could be considered accaptable or it vary from scene to scene? For example, Next Limit says that a pass of 25 is good enough for Maxwell Render. For the optimization I asked for but I didn't received any useful advice. I picked up some settings from other posts. The artificial illumination here is mainly given from meshes with emission material and Corona plane lights.
@Maru: for the bitmap building you are totally right, I didn't liked but were directly asked from the client.