I wrote a post 7 months ago saying "I'm Sick Of It" where I decided to hit the long tones seriously because I was sick of the fact that my high and low notes were undependable. People told me it would take a year before I would get serious results. They were pretty much right, but I have noticed a lot of improvement already. The highs and lows are far more dependable and my tone has gotten much better. Intonation has also improved since I always work with a tuner. I first play the note on the piano, then start the long tone and then check the pitch on the tuner. It's not easy to do 20-30 minutes of long tones. When I get done I feel like I have practiced for hours.