It is quite difficult to over-drink, as obviously it makes you need to go to the toilet! It is possible to drink so much that you dilute the salts in your body too much, but as long as your urine is a pale yellow colour (it will be dark yellow in the morning, due to not drinking and losing water at night) and you're not always thirsty, you'll be fine.
Over-use of your bladder shouldn't be a problem, but unnecessarily holding on to water can be. If you need to go to the toilet, then you should and empty your bladder completely.
You just need to make sure you're sufficiently hydrated. Even slight dehydration can cause a significant drop in performance, be it sporting, working, trying to think. If you're thirsty, then you're dehydrated and need to drink (water or other fluid). Obviously you also consume water through what you eat.
You should be able to judge for yourself how much you need, I wouldn't worry about the actual amount too much. For example, if you're doing a lot of manual work or sport where you're losing water through sweating,you'll need to drink more, but also replace lost salts. If you're in an air-conditioned envirnment, you'll often find you need to drink more as the air is particularly dry. Similarly if it's dusty etc.