i dont know about standard, but with advanced, change IS the area of study...
but, back to the question, my teacher recommended that we at least fill up one of those hsc booklets (however i dont know how much is the accepted standard length)
Yeah I agree with the others Quality Not Quanity , Now I think about it is probably doesn't really matter what essay you are doing, you should try and get about 6 pages for each of them. Just from experience though i tended to write more on changing perspectives than say Macbeth for example.