Sweating is a very individual thing. It is that heavy athletes, faster athletes and hot conditions will cause higher levels of sweating whilst lighter athletes, slower athletes and cool conditions will cause lower levels of sweating. Of course, you could be somewhere in the middle (for example a light, fast runner on a moderate day). There are also individual difference. Some people are salty sweaters whilst others are not salty sweaters. Using this information you can make an informed assessment on how much hydration you may need.
We sweat water and salt, mainly. This means that hydration should contain both water and salt. Much like with balancing Kcal, it is now understood that you can judge hydration by thirst.
My advice is to spend your sessions getting dialled in with what you need so it is close to fine art on race day. Sports science suggests that everyone would need somewhere between 300 and 600mg of sodium per hour. This is about 1.7 to 2.9g of salt. You can make a good guess as to how salty you are as a sweater by looking at your clothes after training and racing. You will fall into one of the following catorgories and could assume the correct amount of sodium per hour based on how much of a salty sweater you are:
Sweating buckets - drenched and covered in white salt after a race or hard, race effort session, of over one hour - have 600mg sodium p/hr.
Pretty darn sweaty - still drenched in sweat, less white and salty than the most salty sweaters though - have 525mg sodium p/hr.
Fairly sweaty - not completely drenched but wet all over - have 450mg sodium p/hr.
A bit sweaty - a sheen of sweat all over - have 375mg sodium p/hr.
Not sweaty - people can't really tell you have been exercising - have 300mg sodium p/hr.
As the amount needed depends greatly on conditions (how hard you go, for how long, how hot or cold it is ect.). To estimate how much water you need it is worth doing a sweat test in training. Use the five catorgories from above to assess how much sodium in the range of 300-600mg per hour you need. Weigh yourself before and after - 1kg = 1litre of water - but don't forget to account for the weight of water and energy you take on during.
Example: you weigh yourself at 70kg (weighed naked). You do a race pace effort for 2hrs on the bike and consume 1L of water, nothing more. After the session you weigh (naked again) yourself and you are 69kg. You need to account for the 1kg you added to yourself by drinking the 1L of water so you have basically sweated 2kg, that's 2L of water. So you know you are a fairly sweaty sweater due to being not completely drenched but wet all over, so you need 450mg of sodium per hour. Two hours require 2L of water and 900mg sodium which is 1L and 450mg per hour...
Keep drinking throughout your training sessions and races. Drink water to thirst and have the sodium with it. If you are still always unsure on how much water you will have due to an unknown ride or weather conditions for example, you could split the two (water and sodium) by taking on salt tablets and drinking pure water. But if you have a good idea of how much water you need you could add an electrolyte tablet or some salt to your water.
As a general rule for every athlete, male, female, heavy, light, fast, slow etc. I suggest taking your water bottle and simply adding a pinch of salt. Drink to thirst. Do not allow yourself to get thirsty!
To be more precise using the information above, 1/8 of a level teaspoon of salt is around 300mg of sodium and 1/4 of a level teaspoon is about 600mg.
