8:00am till 3:00, with a 30 minute break.
But honestly, if I try to not clockwatch on Rosh Hashona, I find the davening to go quite smoothly and before I know it, it’s over.
I always bring a sefer or even a jewish inspirational book (like Rabbi Krohn’s Maggid books) that I skim through by chazaras hashatz or by the long yoitzros in order to give me a “boost” insted of spacing out.