"Time of useful consciousness" varies greatly among individuals, and even for a single individual under different circumstances. A long-distance runner, mountain climber, or a person who normally lives at a high altitude will have a longer TUC than the average Joe. A cold, fatigue, or other factors can significantly reduce TUC.
For those not used to flying at higher cabin altitudes (e.g., 8,000'), even 100% O2 at a cabin altitude of 30,000' will not necessarily keep you lucid.
Test your personal limits under controlled conditions (e.g., in an altitude chamber) -- NOT as the PIC of an airplane!