hey marina,
It's good your asking, because its sooo important too stay hydrated and also to perform at your best.
The following graph relates % body weight loss to performance and symptoms.
-- 0% -- normal heat regulation and performance
-- 1% -- thirst is stimulated, heat regulation during exercise is altered, performance begins to decline
-- 2% -- further decrease in heat regulation, increased thirst, worsening performance
-- 3% -- more of the same
-- 4% -- exercise performance cut by 20 - 30%
-- 5% -- headache, irritability, "spaced-out" feeling, fatigue
-- 6% -- weakness, severe loss of thermoregulation
-- 7% -- collapse is likely unless exercise is stopped
As i'll put it, you should drink too a plan rather than to thirst. The best way to develop a plan is too measure body weight before and after exercise!Each kilogram of body weight represents a litre of sweat loss,
less than 1% of body weight loss places stress on the heart cardiovascular system and body temperature.
Before Exercise::: you should consume
as large a fluid volume (300-500 mL) as can be comfortably tolerated 10-15mins prior to exercise. For participating in really long strenuous exercise u should fluid load! 24 hours before exercise it may be beneficial. Practice though cause it may cause gastrointestinal discomfort, or a sports drink before exercise will help with carbohydrates,

During exercise:::Replace fluids regularly. Regular consumpsion maintains an optimal rate of stomach emptying and fluid delivery. around 200mL every 15mins while exercising.

After Exercise::: To rehydrate, it is important to consume 1.5 times the amount of fluid lost. If 1 litre has been lost(1%) consume 1.5 litres to restore fluid balance.
hope this helps you

Thanks for the info.
The scientific stuff is quite hard to digest, it took me a few reads to completely understand. I will work out the % lost on weds when I have the very strenuous class as tomorrow (sunday) is a pretty easy one, just technique, so quite easy to gauge fluids. I take it by your reply you have done some sort of sports science as you seem very knowledgeable.
Thanks heeps again!! :D

glad it helped :) Im studying personal training and I did sport science in the past, thanks though have fun in your class