I remember a general rule of thumb being for every 2 pints of water you want to take in a pinch of salt. Obviously that is not very scientific, nor do I have any idea where it came from, but it does seem to have enough value in it to maintain a sense of validity.

In general though, if I am riding in a lot of heat, I will supplement with an Emergen-C packet for every two pints of water I drink.