Remember this, shin conditioning is as much, if not more mental, as it is physical.
If you think "Ow, shit, that hurts", then you will most likely start to shy away from shin conditioning altogether. However, if you take a knock on the shin and think "ok, that hurt, but its gonna get better over time" then you will progress.
There are a number of people in my gym who have trained for about 4 years who still have dellicate shins, because the minute they get a knock. They think "Fuck, that hurt, where are my shin pads!". Just grit your teeth and bear the pain. It is worth it in the long run.
Make sure that after a hard knock you rub in plenty of Thai Liniment (The best brand is available from Ringside.com) and take Calcium tablets too. I take 4 calcium tablets a day, and 6 a day if I have a knock on the shin.
Hope this helps.
As far as how long shin conditioning will take, it depends entirely upon you!