Here is a link to a page I found most helpful but I will summarise my findings:
Like Joel said yesterday:
Your body absorbs only 4-6 oz / 250ml of water every 15-20 mins.
That equates to 32 oz / 1 L per hour - so drinking any more than that through the class is not going to increase your hydration. This stresses the importance of hydrating prior to class rather than relying on the water you are drinking while you are in class.
The best way to work out how much water you need is to weigh yourself before and after exercise and to see what the difference is. You need to drink 2-3 cups of water for every lb lost.
To work out how much water in total through the day you should drink you can use the following formula:
For moderate exercise -your body weight in lbs divided by 2 = # of oz of water required per day.
For strenuous exercise with excessive sweating - thats us folks! - your body weight in lbs equals the # of oz of water req'd per day.
For those in metric 20 oz = 600ml.
Hope this helps :-)