I know hafnium oxidizes in ~ 30 s. at ~ 10^-9 torr.  That may give you some idea. I cant quote anything for the above number ,though.

Dear Sir,

our experiments are, that we sputtered reactive with oxygene to a targed of Fe.

After any 3-4 h we have a good ferrite thin film for a absorption EMC application.

All the best

F. Gräbner

From kinetic theory of gases, you can calculate the number (N) of gas molecules colliding with a surface each second (molecules/ cm2 sec) under a given pressure P. 

       3.51 *10^22 * P   

N=------------------------

        sqrt( M * T)

P is the pressure in Torr, M is the molecular weight of the molecule hitting to the surface and T is the temperature in Kelvin.

Taking oxygen (M=32) at room temperature (293 K) for ~1^-8 Torr, the number of O2 hits on a surface of 1cm2 is approximatelly 3.6*10^12 per sec.

If each hit is efficient (sticking coefficient =1) after 1 sec; you have 2*3.6*10^12 oxygen atoms on the surface (if an oxygen molecule dissociates and gives two oxygen atoms). If your sample has an atomic surface density of 1^15 atoms/cm2, after 137 sec (1^15/2*3.6*10^12) your surface has covered by oxygen. 

Atilla, the theory is nice but please check the calculation! For the pressure of 10^-8 Torr (not 1^-8 Torr), and for the surface density of 10^15 atoms/cm2 (not 1^15 atoms/cm2) I get 24,39 s! Reconsider, please!

Dear Hrvoje,

Thank you for your comments. You are right, I would say 10^-8 not 1^-8 and 10^15 not 1^15. This is just a problem of typing.

But the final result that I got is 137-138 sec and not 24,39 s.      

Yup, you are right! I found my mistake. Its better to recheck! ;-)

Dear Sir,

we have reactive sputtered 6h für 300 nm thick hematit thin film.

Yours sincerely