Number of saves vs lingering clouds of poison doses


If someone were to use either the toxic censer or alchemical wax to create long duration clouds of poison, how many times do creatures within those clouds have to save against the poison dose?

Are they affected once and never again by the same cloud, meaning there’s no difference if they are in it for 6 seconds vs the full 5 minutes or full 1.5-2.5 hours for the wax?

Or would they have to resave against the poison each turn, potentially rapidly increasing the DC and duration of the poison to ridiculous amounts for a single dose? (+2 DC and 50% duration each application)