Originally Posted by housecatnick
It does take 6 months to a year for a painting to dry - to the touch! It takes up to 60 years for the oil to COMPLETELY dry.
Let's not confuse things here - paintings are dry to the touch when they're dry to the touch
That's usually well under a week for thinnish oil layers. The initial 'drying' process then takes some months, enough that the painting can be varnished safely. After that the continuation of the oxidation/polymerisation process can indeed take many decades and this does require oxygen, but the only really significant source for this is from the face of the painting.
Micro-climates in the back of paintings can actually develop more from the small vent holes in backing paper than when the back is enclosed fully by the way.