[Pseudoscience-is-malarkey (OP)]

How fast does smell usually travel, and how far? Does its length (both in distance and time) depend on how strong the smell is when it leaves its creator, and as it expands it just keeps getting more and more diluted until one can’t distinguish it from background smells?

[alancalverd]

Speed will depend on the tail of the distribution of molecular velocities and the minimum detectable concentration, which is why dogs can smell something far more "quickly" than humans - they just need fewer molecules. Sharks are even quicker, and both species can not only detect a smell but also determine its concentration gradient to locate its source. I've seen an experiment where a shark located the source of a drop of blood in a swimming pool within 5 seconds.

[evan_au]

The speed of smell is affected by movement of the medium. Smell travels faster downwind than upwind.
If the medium is not moving, then smell molecules diffuse away from the source. The average speed is slower for larger molecules.

[Eternal Student]

Hi.

   So the replies already given are reasonable.   I'm just surprised no-one took any steps to explain the background.
Smell is essentially about detecting some molecules of type A in the air  (or water for a shark).   A thing smells of substance A  if it releases molecules of type A into the air (or water etc.)  and the creature doing the smelling has olfactory receptors that can react to and detect the molecule of type A.

    So the spread of molecules through the air (or water) is a diffusion process and governed by conventional diffusion dynamics.

   When considering how fast a smell will travel, we then have two important systems to consider:
(i)   A physical system that is just the diffusion of the molecules through the air.
(ii)  A biological system which is the creature doing the smelling.

Does its length (both in distance and time) depend on how strong the smell is when it leaves its creator

    If you consider a situation where the odour or smell has just been created and the odour molecules are being released from one point of origin at a constat rate - then yes.  Diffusion rates should be higher when the concentration gradient is larger.  A creature at a fixed distance from the origin of the odour should have the concentration of molecules in their location rise to a high enough level for detection after only a short amount of time.   For most creatures, the time at which they first smell an odour is not equal to the time when the first few molecules have reached their location.  Their olfactory system requires the concentration to reach a high enough level and that is the important time scale or limiting factor for first detection of the odour.
    Some biological systems, like the dogs or sharks as Alancalverd mentioned, can be extremely sensitive to even just a few molecules in the air.   The absolute maximum speed at which the first few molecules of any new odour can reach a creature that is some distance away from the origin of the odour will be governed by the speed distribution of the molecules (as mentioned by Alancalverd).
    Similar sorts of answers can be constructed for various situations.   The speed of odour detection is always a hybrid of factors due to (i) physical diffusion or movement of the molecules  and  (ii)  the biological system that could detect it.

 

...and as it expands it just keeps getting more and more diluted until one can’t distinguish it from background smells?

    That is pretty much correct.  Sometimes the concentration drops too low for the creatures olfactory system to detect it at all,  no consistent electrical signal is sent to the brain from the nose.
    In some other situations, the brain seems to automatically adjust to a certain level of odour if that has not changed for a length of time and the information is essentially no longer processed as if an odour is detected.  As a human being you may already be familiar with this, when you first enter a new location you may be aware of various new scents (perhaps the air freshener that someone else uses in their house),  after some time in that house you have adjusted and are no longer so aware of these odours (whether they were pleasant or unpleasant initially).  This can happen regardless of whether the actual concentration of those odour molecules has dropped (for example, the air freshener may be a plugged in device and emitting odour at a fairly constant rate).

Best Wishes.

[paul cotter]

An interesting observation about the transmission of odour is the Freiberg thioacetone incident. A laboratory accidently released a small quantity of thioacetone monomer( it normally exists as the trimer ) producing widespread panic over a wide area of the town. The smell was so bad that exposed people vomited and actually fainted.