On a good day I can go up to double C and normally at least G above High C. However I can only scream like this for about 15 min. until my chops are completely dead. How do I improve my endurance in the High register, I`m not concerned w/ playing higher just playing longer in the high register.

lip slurs and long tones

How long can you play within the staff and below it before getting tired? Until you have excellent endurance without loss of tone quality in the low and mid range, you`re not going to magically find it in the upper register either.

I play for hours as long as I don`t go above high C. When I start to go into that scream range my chops tend to give out after a few minutes. For example, I can improvise to smoth jazz for hours but can only play maynard`s Hey Jude once before my chops fall apart.