How to adjust time of useful consciousness for pilots acclimatised to high altitudes?
The simple answer is "they don't". Training will dictate that at altitude, they need to be able to don masks within a few seconds, regardless of an individuals tolerance to altitude. The problem isn't that you have X seconds of full-cognition and then you pass-out, your brain function degrades during that period. It's best to get your mask on and oxygen flowing before doing anything else, and this is how pilots train.

Euphoria and a false sense of ability is a side-effect of altitude sickness/hypoxia. There are numerous videos out there showing altitude chamber tests where individuals are given simple tasks as the oxygen saturation decreases. These tasks can be as simple as those infant "put the shape in the hole" boxes, and it is surprising to see how well the people think they are doing versus how poorly they are actually performing. These two things combine for a very dangerous situation and you, as a pilot, need to recognize that this can happen and to mitigate that as quickly as possible, without trying to take into account your own tolerance (which can change day-to-day) for altitude.

Check this one out, this is a military pilot getting cards wrong, this is a stark demonstration of the effects. These symptoms happen in seconds. Watch how fast his cognition comes back when given oxygen.

Or this one which demonstrates euphoria and the inability to follow directions or perform basic actions.