The smell/scent/odour is due to the chemical nature of the compound that enters our nose. We perceive smell when these chemical compounds react with our olfactory receptors in our nose. I guess you already know this. As Farhan said, the speed depends on various factors. Elaborating on that, It’s basically the diffusion rate of that particular molecule among the gas around you. If the molecules have a high diffusion coefficient, then it will diffuse much faster (but not that fast enough to reach your nose). Again, a molecule can diffuse in many ways.
Diffusivity is not the only factor that governs the speed of smell. Turbulence in the air affects the speed of that particular molecule a lot. Hence, the convective currents play a dominant role compared to the molecular diffusion. This is the primary reason why you can smell stuffs around you. Convection brings you the compounds much faster than diffusion.
So practically, there is no definitive way to calculate the speed of the sound. Interestingly, if we could calculate some of the variables at STP and go by diffusivity, we can predict speeds of different smells relative to each other. As far as I know about this, that’s the closest thing to the speed of smell we can get to.
