I have given myself a headache in the past by drinking too much water, too quickly in hot weather, but I have also got very dehydrated when not drinking enough.
I need to drink more water when I am overweight and unfit. I noticed that my needs reduced by about 50% when I got down to 12 stone. When unfit, I drink about 500 mL an hour, perhaps 600 - 700 mL an hour on very hot days. I add some salt to the drinks I make up for my rides. If I did an 8 hour ride on a hot summer day, I'd end up caked in salt from my sweat and that would need replacing.