They make a save each round they're within the cloud.

If we look at Alchemical Wax, it says (emphasis mine):

A character trained in both Craft (alchemy) and Craft (poison) can infuse one candle’s worth of wax with any alchemical substance that requires inhalation, such as a vial of alchemical pheromones (see below) or a dose of inhaled poison.

Similarly if we look at the Toxic Censer, it says (emphasis mine):

This ornately etched censer has a small hidden compartment under the heat source that can hold 1 dose of poison. When in use, the censer’s heat source indirectly heats the poison, causing it to mix with the burning incense and allow the poison to permeate the air, filling a 20-foot radius centered on the censer. The poison’s type changes to inhaled, and it is effective for 5 minutes before it is entirely burned off.

This means that both of them follow the rules for inhaled poisons, which state (emphasis mine):

Most inhaled poisons fill a volume equal to a 10-foot cube per dose. Creatures can attempt to hold their breaths while inside to avoid inhaling the toxin. Creatures holding their breaths receive a 50% chance of not having to make a Fortitude save each round. See the rules for holding your breath and suffocation in Chapter 13. Note that a character that would normally suffocate while attempting to hold its breath instead begins to breathe normally again.

So we can see that a creature within the area of a poison has to make a fortitude save each round in order to see whether they get affected by it, although they can hold their breath in an attempt to mitigate the number of saves they have to make.

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To address your other point, according to the poison FAQ blog, they would indeed have to deal with increased consequences each round they fail a save while in the cloud.

1. Whenever a character is exposed to a poison (regardless of method), that character gets a saving throw to negate the poison.
2. The saving throw DC is increased by +2 for every active dose currently affecting the character. Poisons that were cured, have run through their entire frequency, or were negated with a successful initial saving throw do not increase the DC.
3. The character must make a saving throw against every poison affecting him on his turn, but may make the saving throw at any point during his turn. If a poisoned character delays his turn, he must immediately make these saving throws. They are not delayed.
4. Unless the poison has an onset time, the character takes the effect of the poison every time he fails a saving throw against the poison, even when additional doses are inflicted.
5. The total duration of the poison listed in the frequency only increases by half the original duration and only when the initial saving throw against a dose is failed. If the initial saving throw is made, the duration is not increased.
6. If a character is exposed to multiple doses of inhaled and ingested poisons simultaneously, only one save is made at the higher DC. If the save fails, the character is subject to all of the doses, but still only takes the effect once for the failed saving throw. If the save succeeds, the character avoids all of the doses.
7. Finally, if the character is exposed to a poison that is similar, but not the same, such as having a slightly different frequency or DC, it is treated as a different affliction that is tracked separately, even if it has the same name or other identical entries.