I skinned my left ring finger on Christmas eve and while to all intents and purpose the finger is healed however I still can not play the guitar with this finger - the pressure against the string is too much to bear. Will the skin get thicker and the sensitivity reduce over time and how long will it take?
If it was the pad of the finger that you skinned, unfortunately, it will take quite a while.
There are a lot of nerve endings in the pads of the fingers. When you took the skin off, you basically exposed those nerve endings. Scar tissue is not a very good substitute for the real thing, but, it will, with time become tough.
Just like when you first start playing the guitar, you had to build up the calluses on the pads of the fingers, so they didn't bother you, you will have to toughen up the scar tissue on the pad of that finger.
It will eventually toughen up, but it is going to take quite a while.