I think you both make good points. Part of it is stamina like Quentin said, although a larger part is technique. Back when I used to just do screaming with no technique, I was able to build up some stamina but it would only be to be able to sing about 40-60 minutes, never multiple hours. And my voice would always be worse the more I sang. Nowadays the more I sing, the BETTER my voice condition is. Nowadays, the more I sing in a day the better my voice is. If I sing 5 hours, my voice is at its best at the 5th hour...if I sing 8 hours, it is at its best on the 8th hour. I just feel things working easier and better.

Of course I am taking mini breaks here and there. What tends to happen is I feel a bit of fatigue set in so I take a break for 5-10 mins, come back and the voice is smoother and stronger and requires less effort to power.

To answer your question Owen...almost no one has perfect technique (it is a lifetime development), which means that we will make small mistakes here and there that may cause a bit of stress or fatigue. The more experience you get, the less you make these mistakes or the less intense these mistakes will be. So what I'm saying is, you will find you can sing for longer and longer periods of time as you get better simply because your technique will be more proficient.

As for grit, it is definitely something I wouldn't want to sing with before a warm up. For me, I feel that if I have to artifically force grit in the sound, it never feels healthy to me. When I can get grit by simply trying to sing more intensely that is when it feels very very healthy to me, I feel it as a layer on top of my clean vocals. And this is how I choose to teach grit to singers...by just getting them to become stronger at increasing their intensity until the grit naturally comes out by itself. When it is done this way I have only noticed positive results.