I generally steer clear of threads like this due to the toxic nature of the debate that usually happens after a few posts. The only reason I am posting is because CD has asked the question, and I suppose there will be quick/direct moderation when it goes off topic to "beatings/pile-ons"
This is a very difficult question, and is very controversial based on each divers training (time period) and experience. I was first certified for 130' as the sport diver limit. Others have been certified under different limits, both deeper and shallower based on when they were certified... also years of testing and experience (based on EAN21).
I have realized narcosis at 60' and beyond depending on many factors in the past. I have not felt narcosis at 30', but I do understand that it is there... But, the subjective question here from my perspective is "at what depth do you believe deep air will assist you in making proper decisions in a timely manner?"
I will admit that my deepest dive on EAN21 was to 255' with a plan at 165' in Cozumel. At the time I was recently certified as an instructor, in shape, and I was diving with a group lead by another instructor.
The plan was for the "group supervised by the lead instructor" to stay with the DM, and the lead instructor and I would dive our plan to 165'... At 145', I checked my gauges, with a slight buzz in my head... I began to level out... I looked down and saw fins only. At 165' my head was ringing and it was all I could do to concentrate on my gauges and my bottom time. I was on the ledge and I just saw the fins of the other instructor...
This is before dive computers, and I knew I had my dive tables in my BC, so I followed in an attempt to catch those fins... I made a decision right at that moment that I would not go beyond 240' (PPO 1.6)... I caught his fins at 255'. He turned to me and indicated it was time to go deeper... I shook my head no, and gave him the thumb up... He turned and started down. I grabbed his fin and began ascending via kicks upward. He tried to kick away, but I had his leg locked under my arm...
Shortly, he turned to me and gave me an "OK". I stopped and let go of his fin to see what would happen (I have no idea what my gauges read at this time, my only purpose was to ascend at a safe rate of speed, following the wall up)... But, as I let go my only thought was "If he starts down again, he is on his own"... I remember feeling relieved when he started upward... My thought "OK, we blew our plan... now the rest of the dive is a hang when we get there"...
We ascended to where the others were drifting at 90'... He stayed with that group and I kept ascending, since my tables didn't include 255', I did an extended stay at 20' and the rest of the tank (to 200 psi) at 10'... I dove the next dive to 55' and kept it conservative. The remainder of the week, I took control of the dive group and no one dove deeper than 130'... The lead instructor for the group I was with was in a wheelchair after day two, and he blamed it on a Cozumel hooker
All dives after that have been 200' or above. I regularly dive 130' and above...
Based on my previous training and experience, the fact I am much larger than I was at 21... and my exercise regimen... I would say my personal idea of "at what depth do you believe deep air will assist you in making proper decisions in a timely manner based on narcosis" at the PPO... 1.6... Or what ever emotional state I am in at the time I make the dive to that PPO...
Basically... It changes with every single dive you make!