When ever anybody talks about the drying time for oil paints they always put it into the same context as we talk about the drying time of watercolours or acrylic paints, which is wrong really as the process is completely different. With watercolour and acrylics the paint dries through evaporation, which basically means the water in the paint is drawn out by spontaneously turning from a liquid into a gas, this process allows the paint to harden. The hotter it is, the faster this process happens, which is why it's a bad idea to paint outdoors with acrylic in the summer, as your paint dries almost immediately.
How many times have you heard someone say, "It takes nine months for an oil painting to dry" no wonder a lot of people won't touch oil's, especially beginner painters, I'm not surprised really, can you imagine having a wet canvas sat in your studio for nine months, well the good news is that this myth is actually untrue, well it's more of a misconception than an untruth, It's quite hard to say when a painting will be dry because there are various factors to consider, like the colour of the paint, as brighter colours don't dry as fast as the earthy colours, also how thick it has been applied, but as a general rule an oil painting will be tack dry the next day, for it to be completely dry it will take a few days, possibly up to a week. The nine months is true, but it's not for the paint to dry, it's what I like to call "Cure" it takes approximately nine months for an oil painting to cure, to properly harden, and this is where the confusion lies, it isn't nine months for the paint to dry, its nine months before you should varnish your painting. You shouldn't varnish until it is properly dry because oil paint dries through oxidisation.
With traditional oil paint, there isn't any water in the paint to evaporate away; the water element of the manufacture is replaced with an oil based substance, usually Linseed Oil which also doesn't evaporate away. What actually happens is when your painting is exposed to the elements, the oil reacts with oxygen in the air which causes the paint to harden through oxidisation? So as long as the oil paint is exposed to oxygen it will carry on with the curing process until it is completely dry. The problem with varnishing your oil painting before it has sufficiently cured, is you are cutting off the supply of oxygen to your painting so it can't carry on with the oxidising process. What would happen is your oil paint will eventually dry but it would take a very long time to finally harden and there's a very good chance the coat of varnish you have applied will crack, this is because the top layer of varnish will dry first, and the under layers of oil paint which will dry at a much slower rate, will move and contract so it breaks the hardened varnish as it dries, this is a possible reason why sometimes an oil painting has the look of snake skin, where there is cracks all over the painting.