A synthetic brush uses 1/3 to 1/2 the soap of a boar.
That generalization is far too simplistic. The diameter of the knots, the loft of the bristles, the packing density of the bristles, the loading time, and the loading pressure all enter into the equation. I have a 35 year old boar knot with a small diameter and low loft. I also have a Omega professional boar with a huge knot. They load soap and generate lather quite differently. I have a couple of 26mm high density synthetic knots that will hold as much soap as the Omega boar.