I can still sit in a stand dawn till dusk, and almost 56 years young. Providing the seat is at the right height that my knees don't lock up on me. If the seat is just a half inch low my knees will lock and legs begin to cramp. When this happens I'm lucky if I last 2 hours. And getting down is a real pain in the a** too.
Standing I'm good for 3-4 hours in morning and again in afternoon.
Like to still hunt but generally where I hunt it is just to heavy of cover to move through and see anything, so taking a perch is most desirable.