Actually, I have ridden 3 centuries this year. My average time for the three rides is 6 hours, 45 minutes. My younger brother normally rides a century in less than 5 hours. I am not sure if there is an accepted standard time for a "good" rider, average rider, or even a mediocre rider as I strive to become. I have asked my brother if he is thinking of entering some local races. He says every time he considers such an adventure he rides with someone that does race some. He says he is then taught how average he is and all thoughts of racing are dashed. As it has already been suggested, I think probably the best thing is to time yourself and try to improve on that at whatever distance.