It would all depend on your personal ability - you could do it in 15 hours, or it may be 20... either way, you've only got 6 months to get the training done and get the rating issued.
I did mine in minimum time, but I had a very good instructor who had also taught me during my PPL and Night Rating training.
As well as the 15 hours minimum flying time, there is also another written exam to pass and of course, the IMC Rating flight test.
It has been a very useful rating to me and has got me out of some situations that I could see could become very unpleasant! I believe it helps to make you a better Pilot too, as you have to fly pretty accurately - My instructor knows that I intend to progress to CPL and IR, so he made me fly to IR standards, which I can only see as being a good thing!
Good Luck with the training and if you set your mind to it, i'm sure you'll have the rating in your hand in no time
P.S. Congratulations on gaining your PPL, I still remember how nervous I was on my Skills Test!
