Sorry for the wait been building a new quad a 666 mm frame ,God thats a hard one i dont know many that can fly that long , the bigger the battery the more its weight so you dont gain much if any thing , and after about 15-20 mins your start to loose your head and will have more of a chance for a crash,DJI phamtom is one of the best and it dont fly for that long,My mate gets 19 mins with his set up ,so all you can do if your going to fly FPV and total flight time is 20 mins set your timer to 10 mins then that gives you time to come back, he has same frame as you so look at his set up and try to match it the spec is in the first part of his video
HAPPY NEW YEAR JOE