gym workouts - hydration

I work out 6-7 days a week and since I'm a trainer/instructor I spend more time in the gym than the average bear.  While it's not pretty or ver girly to admit, I sweat the entire time.  Someimtes that 2-4 hours a night. 

I can't seem to get my hydration up enough.  Yesterday during an intro to step class yesterday afternoon I got so dizzy that my brain became foggy and I was having trouble following the simple choreography.  I know it was hydration because by evening I was fine...of course that was 2 bottles of Powerade Zero (cannot tolerate sugars at all) and 1 protein meal replacement later.

I log my intake on a regualr basis using fitday.  I usually hover around 100oz of fluid.  How much more do I need?  How much do you consume? 

Are you sure its hydration and not low blood sugar?  I had (no more band) issues with low blood sugar and running/weight training since I was having problems eating carbohydrates.  Do a search online for Hypoglycemia symptoms.