At altitude, effects of hypoxia are measurable in just a few seconds, HOWEVER, that doesn't render you dead/unable to do anything.
For a good example of how you get 'loopy' at altitude, go into youtube and search for "Kalitta Hypoxia" for the ATC recordings of a Kalitta flight that decompressed without the pilots fully grasping what was going on. Back at 16,000 or so, you'll hear how much more coherent the pilots are vs the 30,000 decompression