Atleast more than 2 liters everyday should keep you hydrated. But There is no exact amount fixed for everyone. if you ask why because there are some factors that play vital role on how much fluids your body needs.. The first factor would be "Humidity".

See, on a humid day you sweat a lot.. That drains lots of fluid from your body and obviously the more you sweat the more you should drink. these circumstances are limited to summer times. but in winter humidity is less and you sweat less. So in that season you can get away with drinking less fluids.

Second factor would be the physical work amount. If you have to use your body muscles in any kind of work than you burn through more fluids. So the more you work the more you should drink..

Now the third factor is you body size. This is the determining variable about how much you should drink.. See a person has 55-64% water in their body. so if you weigh 100 Kilograms than 60 kg is water. Now everyday you lose from 2-8% of that water from pissing and sweating and the amount you lose is determined by the previous two factorsn if you from that 60kg lose 2-8% then you can lose 2-5 liters of water a day.. Now you counting in all the variables do the math and figure out how much you should drink..B