I'm am hardest in the morning and less so at night, but it still stands up at night. I find when I have sex after hours of drinking it improves my stamina but gives me one of those hard ons that's hard enough to penetrate but can't stand up on it's own, it is still thick but hangs down. I've found that fucking a girl with the really hard one will help them to orgasm faster than with the droopy one, even though the size is the same and I can go longer with the drunk one.
I know that my friends with smaller cocks than me are harder than mine on average, I guess that' one drawback of a large penis.