I honestly can't answer, it varies so much and only I only really know the principles of gaining weight, not average statisics etc, at 17 I wouldn't worry to much about trying to gain muscle any way, trust me your still about a year away from starting to physically mature...

The past 6-8 months I must have put on about a stone in muscle, but I don't think thats a very typical thing to do, I just suddenly started filling out when I was about 18 and a half, before then I was really skinny, weighed just over 12 stone and I was and still am ( : ...) five foot eleven. No I struggle to stay at my fighting weight which is 12 stone 12 (my normal weight is now more like 13 stone 4.... )

you just shoot out like that when you reach a certain age...

Unless your trying to become a body builder theres no need to try and make massive weight gains, eat and live healthly, train hard and sensibly and just fight at what ever weight your natural at. Learn about more about your body, the why it feels and reacts to training and then start planning which weights you feel strongest at etc.