Jeeesus.....buddy.....to my knowledge, most guys have no trouble coming within about 5 mins of starting to masturbate

8 hours man!!.....I'd be wearing off skin going that long.....

Does it take the same amount of time with a partner?
You say that *it gets harder before I cum*....does that mean you don't really get an erection for several hours before?
If the answer is yes to the above questions mate.....I'd say there's a gotta'be some sort of underlying medical problem here.....have you talked to your doctor about this?