That's kinda arbitrary
Drinking about 32 oz at the end of the night is usually good for dealing with most hangover symptoms. But not all, so here's a list of what causes them:
1) Dehydration- the important thing is to drink water at the end of the night, when you are not adding more aqua vita into your system. During your session is fine as well, but at least in my experience drinking water at the end of the night/morning is essential. This is also the leading cause of a good host of hangover symptoms.
2) Lack of restful sleep- alcohol wrecks your sleeping ability. You may "pass out" but you aren't really getting the actual type of REM sleep you need. Since I was up until five this morning, I believe this was a huge contributor on my part.
3) Lack of various electrolytes- as you get dehydrated, you lose some electrolytes which will leave you somewhat sore the next day. That's why I drink Gatorade at the end of the night. Not too much, and you still have to drink water otherwise you will get dehydrated like a chump.
4) Various characteristics of what you do drink: Tannins, Red Bull, other stuff you mix into your drinks will cause various problems as well. I have these last because they normally do not cause too much trouble unless you do something stupid.... like drink massive amounts of red wine, for example.
Having a cat to cuddle with is also helpful. Having an old dog with mucus oozing out of her empty eye-socket is not.