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Monster Slayer Stan, we are in dire need of your help! The caves in our nearby mountain are filling up with a dense but non-toxic gas that is protecting a monster's nest. The monsters are weak but are hiding deep in the caves. We need you to go in and destroy their nest.

In another world building answer it states that people who breathe in argon have a risk of asphyxiation. With that in mind, how soon would Stan notice the environment was 95% argon? After breathing in that much argon and escaping, how long until Stan can breathe properly again?

Stan will not notice he is suffocating. He will pass out and die.

Our need to breathe is triggered by accumulation of CO2. High levels of CO2, even with adequate oxygen, will provoke the sensation of shortness of breath and suffocation.

In contrast (for normal healthy people like Stan), if there is not high CO2 you feel no particular need to breathe from progressively lower levels of O2. When your O2 gets low enough you just pass out. This is called shallow water blackout.

https://en.wikipedia.org/wiki/Freediving_blackout

Shallow water blackout which occurs when all phases of the dive have taken place in
shallow water (i.e., where depressurisation is not a significant
factor) and typically involves dynamic apnea distance swimmers,
usually in a swimming pool. The mechanism for this type of shallow
water blackout is hypoxia expedited by hypocapnia caused by voluntary
hyperventilation before the dive. Blackouts which occur in swimming
pools are probably driven only by excessive hyperventilation, with no
significant influence of pressure change. This can also be
described as constant pressure blackout or isobaric blackout.

This happened to me once from breathing helium to make my voice high. I blew out my air and breathed a lungful of helium and talked squeaky. When it was gone I took another breath of helium. Then I came to under a table.

In this circumstance, Stan has argon instead of helium. Argon does fine clearing CO2 out of the lungs and so he will not notice anything wrong. It is a pretty good defense for those monsters.

With 95% argon (oxygen can be 5% or can be 0%), Stan would likely lose consciousness within a minute - without feeling any suffocation alarms. If he is somehow brought into the normal atmosphere, it will take him a few minutes to get back no normal, assuming there was no permanent damage to his nervous system.

Inert gas asphyxiation is a more sneaky killer than typical (smog or excessive carbon dioxide) asphyxiation because a person does not feel much until losing consciousness.