The short answer is that for normal people it should take about 2 to 3
hours. There is an easy way to find out. Drink a litre of water, and see
how long it takes for your rate of urine production to go up and then go
back down.
But it depends on several things.
First, the water has to be absorbed. For example, if someone has really
bad diarrhea or is vomiting, the fluid won't be absorbed.
Second, it depends on what is in the water. If it is pure water rather
than water with salt in it, the pure water will be excreted faster than
salt water.
Third, if someone is dehydrated, say, was playing soccer for two hours and
sweated out two more litres water than he drank, the fluid would stay in
his body and his rate of urine production will stay really low until he
drinks more. And if one lost two litres of water and drinks three litres
of water, it will take longer for the rate urine production to increase
than if he is not dehydrated.
Forth, it depends on the time of day. Usually, people's rate of urine
production (assuming identical rate of intake of fluid, like if one has an
IV) in the middle of the night and increases around the time he wakes up.
Finally, it depends on the state of health of the person. IF a person has
kidney disease, the urine production might not increase as much. ANd if a
person has heart disease, the fluid may build up in his tissues instead of
being excreted.
The reference is a paper where students drank water in the morning and
determined how long it took for the water to be excreted. In this paper,
it looks like they urinated out about 400 or 500 ml of water over about 2
hours, before the rate of urine production slowed down. For a liter of
what is needed, it should take a little longer.
