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The rules for poison are a little unclear. A question arises with inhaled poisons it says that multiple doses may be administered at one time and that they take a 10ft square of space. What is not addressed is how long they last and do the doses in the air get used up or do they persist for new victims.
It is not explicitly addressed so it is up to the GM to decide.
In general, poison gas just hangs around. Even if breathed in, the cloud will not be diminished significantly. This stands for clouds of poison, just big billowing sickly yellow-green death-gas. Unless the specific spell or trap or description of the effect states otherwise (such as a duration for a poison cloud spell).
However, you could have some fun with specialized traps that release just a puff of gas that is worth a few inhalations, does not expand, and dissipates quickly.
I believe the idea is that for each vial of poison used in an area, you get a cloud that can deliver one dose. The idea being that you can stack poison clouds. So for three doses, he'd have to toss three vials into the same area. While not expensive, There are issues of time and transportation to consider.
Frankly, I'd look for a few more opinions on this. I can't find rules on inhaled poisons beyond the blog's passing mention.
From what the blog says, the only benefit of hitting someone with multiple doses of inhaled or ingested poison is to increase the save DC (+2 per additional dose). Damage still remains the same and saving against it once removes all doses.