During my many hospital admissions for dehydration, I'm certain the Doctors advised that to ensure I remain hydrated, I should ensure I urinate NO LESS than 3 times a day, aiming for at least 3 or 4 times per day as a minimum.
Apparently, it's a more of a reliable guide to measure a safe fluid output rather than intake, as there's too many variables, such as size, age, activity etc.
Also, one persons fluid intake can be very different to someone else; where as the urine output, is the fluid the body no longers requires, regardless of how much fluid has been consumed.