How fast marine velvet strike?

[Coleus]

I understand that once the fish got marine velvet, it can kill the fish very fast but how fast can it get to the fish once it introduce to the tank?

I really @#$@#$@# up, i picked up a clam from LFS today and forgot to quarantine it and put it right into my main display and now i am worry. No sight of velvet yet but got a feeling of something is not right.

Anyway, anyone know how fast velvet get to the fish? Also how do you tell even if I quarantine the clam but my quarantine tank does not have any fish in it.

Thanks