Smells are just chemicals. The speed that chemicals diffuse in the air is predicted by Graham's Law when you compare two gases.
Use oxygen as your standard gas.
The ratio of the speed of diffusion on the smell compared to oxygen is equal to the square root of the inverse of the mass of the oxygen molecule divided by the mass of the molecule of the smell.
The molecule of sulfur has 8 atoms of sulfur in each molecule, oxygen has two atoms per molecule. The mass of sulfur is 32 and oxygen is 16. The mass of their respective molecules are 8x32 and 2x16 or 256 and 32. Take the ratio of the two, oxygen over sulfur is 32/ 256 or 2^5/2^8 or 1/8. Take the square root of 1/8 is 0.354.
So the speed of the smell is about 1/3rd of the average speed of the molecules of oxygen in the air.
