| I know you didn't want this answer but I think the answer is indeed "as high as possible". If you're asking what's necessary then that's a different question.
I think the average amateur should strive for a strong C and D. Most professional players have a fairly useable high F and G but keep in mind that their C's, D's, and E's are perfect, everytime, after a 5 hour gig. Lead players are not generally asked to play any higher than a double G but it has to be as big as a house and I suspect that even if G is the top of their useable range they are able to play with some consistency up to double C's, D's, and higher. True high note specialists are often able to go to triple C if not higher.
My question to you is why ever stop trying to get higher? Depending on what your needs are you might stop worrying at a certain point but I think it's wise to always try to expand your range(as well as expanding dynamics, articulations, etc). The higher you can play the easier your lower range will be and the more consistent it will be. Remember years ago when you were struggling to play G's on top of the staff after a one hour rehearsal? I know I do. I bet you haven't missed one of those in a while, even after many hours of playing.
Sorry for all the rambling, here's the short version... when will you be able to play the lead book? Well, range wise, I would say you need a double G from mf to fff that lasts several hours. I expect when you have that range and power you'll be squealing at least up to a C. |