Apollo spacecraft used a pure oxygen atmosphere while in space and an atmosphere with 60% oxygen at launch time. This chapter describes how the atmosphere composition varied from 60% to 100% oxygen in the firsts hours of flight. This can be seen on this graph. While getting back to Earth, the inside atmospheric composition should haveequalized with what the outside atmosphere would have been when the hatch was opened. I imagine this change must have been progressive.
How many times did it take to change this composition from 100% of oxygen to the Earth atmosphere levels (about 21% of oxygen)?