Smells are generally caused by particles or bacteria. The sense themselves don't physically move and require external forces to move them such as wind or gas. The further you get away from the source of the smell, the less concentration of the particles that caused smell as the particles have been dispersed by the transport medium.

Take the example of breaking wind, the moment you break wind, the smell is most potent as the concentration of the sulphur particles in the methane gas is at its peak. The methane and sulphur particles disperse into the air, causing the smell to disappear.

As Toni's answer mentioned, The smell itself does not have fixed speed once the particle has entered the nose, it triggers an electrical impulse in the brain causing a smell response to be triggered.

Nobody would think that smell would actually have a speed and in fact, it doesn't. If the speed of light is 186,282 miles and per second and the speed of sound is 761.2 miles per hour because they travel on air, they are waves of energy.

However, the sense of smell doesn't travel through waves of energy but through electrical signalling and this information is transformed into neural impulses that reach our brain. In short, travels in the air so figuring out the speed of smell requires figuring out the speed of air.