Trapped in a well, dying of thirst. How long do I have?

A typical person loses 1.5-2 liters of water a day through sweat, urine, evaporation across mucous membranes, lungs, skin etc. if they are not exerting themselves overly or thermally stressed. This loss could be minimized if a person is cool, completely at rest, and is in a relatively moist environment (which would lessen evaporative water loss).

I believe there as a prisoner once who was accidentally left in a holding cell for 18 days without food or water, and he survived (don't know what kind of long-term damage there may have been to kidneys and other organs). He was able to lick condensation off the wall, though. You might be able to claim something similar for your character if your well is still a bit damp. He could suck a bit of moisture off the walls or out of the soil, and the relatively humid air in a still-damp well would be less dehydrating that typical dry, desert air.

Without any access to moisture at all, the maximum, under cool but dry and shaded conditions with no exertion, would likely be no more than a week, probably less. A fatter person might be able to last a little longer than a very lean person, as having more stores to break down would likely yield more metabolic water. A typical person makes 300-350 ml of water a day in their mitochondria via metabolism. This is not enough to survive , but if a person makes more than the usual amount, it could possibly slow their death a bit.

He could drink his own urine to stay hydrated like the hiker depicted in the movie 127 Hours.

There's the old 3-day rule for going without water, maybe a little longer than that. If I were reading your story I'd have a hard time buying him surviving more than five days or so.

Auteur said:

He could drink his own urine to stay hydrated like the hiker depicted in the movie 127 Hours.

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Drinking urine may possibly work initially, assuming the person was well hydrated to begin with. A well-hydrated person's urine is less concentrated than their blood plasma. However, the urine produced after a day or so of not drinking, or after drinking the initial urine, would be much more concentrated. Human kidneys are limited in their ability to concentrate liquids and excrete salts and wastes without losing a lot of water (it's why we can't drink seawater), and if a liquid is more concentrated than blood plasma, it's actually dehydrating to drink (you lose more water getting rid of the salts and wastes than you gain), and you are introducing increasingly concentrated nitrogenous wastes into your body as well as salts.

Recycling urine might give a person an extra day or so if they start drinking it immediately, before they are dehydrated. There was a man trapped after an earthquake who survived six days, and he did drink his own urine. I've heard, though, that military field manuals say not to drink urine, probably because they assume a person will only try it when they are already pretty seriously dehydrated, and thus making urine with concentrated salts and wastes.

Of course, this assumes he has something to catch is urine in so it doesn't simply sink into the dirt. If one wants to display desperation and emphasize grittiness, showing him trying to suck urine out of the dirt might be a thing.

The reason I know some of these things is I teach anatomy and physiology courses, and these kinds of questions come up sometimes when I have a fun group of students.

This article discusses some survival myths.

https://www.outdoorlife.com/blogs/survivalist/survival-skills-5-myths-dehydration/