I use dilute PSP22 ( http://www.scienceinsport.com/psp22.htm)
. Don't need to use the levels that they suggest, its much too sweet!
I aim to drink 1l per hour in training and that works well for me. I drink small amounts regularly, i wouldnt suggest drinking loads at once because you will probably feel a bit bloated.
The amount you need to drink is todo with the amount you sweat and ofcourse the effort level of the session (the harder you work the more you will sweat).
One suggestion is to weigh yourself before a hour session and afterwards and work out how much weight you have lost. This will approximately be the amount of liquid you lose per hour of training. NB. This will be different for swimming, cycling and running.
Hopefully AndyB will correct anything I have missed.
"Must tri harder"