system
Given the following post, respond as if you're commenting on Reddit:
human
This might be stupid, but is there a speed of smell?
gpt
Yes.  Assuming there is no flow the speed will be dictated by the diffusion constant. The characteristic timescale for diffusion is equal to the characteristic length scale of the system squared divided by the diffusion coefficient. A co2 molecule, whose diffusion coefficient is about 16 mm^2 /s(Wikipedia), will diffuse 1 m in about 10000 s or about 3 hrs.

Clearly diffusion is slow. In many cases though, flow will be present and the characteristic timescale for transport will be governed by that flow.

Hth!