There are quite a few stories about people trapped or otherwise incapacitated resorting to drinking their own urine in an attempt to stave off dehydration, ignoring the long term consequences, my problem with this idea is that the main cause of death when dehydrated is kidney failure, resulting from the buildup of urea and uric acid when the body can't package those chemicals with water to execrate them. Because of this, I'm tempted to assume that drinking your urine (without somehow purifying it) is a futile action as you are consuming both the water and the exact chemicals you need to remove with the water. I'm however not confident with this analysis, so any sources on the subject would be appreciated.
Drinking urine could help you stay hydrated a bit longer, maybe a day, but only when you are well hydrated at the start. You could get about 0.5-1 liter extra water this way. This doesn't work anymore when you start drinking urine the 2nd or 3rd day without water - the amount of urine will be very small and will be too concentrated at that time.
A man, drinking his urine has survived for 6 days under the ruins of the earthquake (Slate.com). However, people have survived for as long as 8 days without water or drinking urine: a 7 and 11 years old kids under the ruins of 2010 Haiti earthquake, and a 35 years old woman trapped in a lift.
According to Guyton and Hall Textbook of Medical Physiology:
...to excrete an average amount of waste products, let's say 600 mOsmols, your kidneys need to excrete at least 500 mL of urine, because they have a limited ability to concentrate urine.
If you drink 1 liter of urine with 600 mOsmols of waste products, you will need to excrete 500 mL of urine to excrete those 600 mOsmols and additional 500 mL to excrete 600 mOsmols from your normal metabolism. So, drinking 1 liter urine forces you to lose 1 liter of water, which means no water gain.
If you are very well hydrated at the time of first urination, your urine will be more diluted, so containing less than 600 mL of urine (minimally about 50 mOsmols/kg according to Wikipedia). Drinking such urine can provide net hydration, but your next batch of urine will be much more concentrated and will provide much less hydration. At the point when the kidneys need to excrete more water to eliminate the waste than you have drunk by urine, you will be in the negative water balance.