How long does 6 gallons of hot water last?

That's a really hard answer for us to give to you... It all depends on so many factors ( how hot do you like your water, how cold is your cold water, can you run on both gas and electric??? Do you use low flow shower heads, can you stand taking a navy(ish) shower a rinse, shut off the water, soap up, rinse...) It goes on and on. I won't even guess. I can only suggest that you experiment if you can and see for yourself. Maybe one of the other folks can give you a better answer than I.

On edit - I can add that. The wife and I both took showers a few minutes apart and neither of us ran out of water. Can't remember if we were using just electric or electric and gas... It won't last forever - but you can get a good shower in if you want in my still somewhat limited experience.

I get in there and get it done, I have not ran out and I just use electric, i have never turned the gas on with it...:B