masturbating is the cause i would say.
you start around 13 or 14 years old, while living with your parents. the idea is to not get caught. so you try to get the job done as soon as possible. after several years this habbit will lead to premature ejaculation.
for me personally it varies quit a bit, i have no idea what my deal is. sometimes it takes me all of 5 minutes, others it takes a good 40 to an hour.
but without getting too graphical, what i have learned to do is wait 5 minutes for the poor old bugger to have a rest and then go at it again. if the first one was only a 5 minute job, the next one will take 30 or 40 at least for sure.