General rule of thumb that our medics used to pass on to us when my army unit was in the field on hot days.

When spending time outside during the summer months, you should urinate regularly (at LEAST once every 2 hours) and the urine should be clear.

Dark yellow urine, or a lack of need to urinate, is the earliest sign of the onset of dehydration.

For most people, this involves going through a quart (roughly the same as a liter) of water every 45 minutes.