To mount a 35lb wood framed antique mirror, I got 4" long concrete screws on recommendation from someone. They're 200lb load screws. The wall is above a fireplace and has half an inch of plaster followed underneath by concrete or possibly brick (we're not sure). Looked more like concrete, not really red.
So that's about 3" that are in the concrete, 5/8" of that is only in plaster and there's about a 1/4" sticking out and kind of hovering in a slightly oversized drill hole. I've included photos.