If there is initially 1.334 m o2, how long will it take for the o2 concentration to reach 0.443 m?

To answer, we take it that the rate of the consumption of O2 in this item is directly proportional to its concentration in first degree such that, 
                       -dCa/dt  = kCa
Solving,
                    -dCa/Ca = kt
                   lnCa - lnCao = -kt
Substituting,
                   ln(0.443) - ln(1.334) = -k(t)
                            t = 1.1023/k