Hi hghbuzz,

People are unique in their capabilities in this area. Both Trimnut2 and Fonty are able to dry fast comfortably for 7 days and still have a moist mouth at the end of it, demonstrating an amazing ability to draw water from their fat cells. Myself, i can do this for about 16 or sometimes 24 hours before moderate and persisting thirst arises which includes a dry mouth... i am thirstier after 20 hours than these guys are after 7 days!

After about 48 hours i step into heavy persisting "want to jump out of my skin" thirst and the earliest signs of minor dehydration such as mildly drying skin.

Since i enjoy throwing myself into my own experiments (for the betterment of the world :p ), i very recently went 3.5 days with no food or water, but i believe i went at least 1.5 days longer than i should have (or likely ever will again) and i am pretty sure this was the cause of my nausia over the next 10 days as i continued on water-only before i finally got better and was able to break my fast feeling great after a total of 17 days.

Heat, humidity and activity levels are going to be big factors in how long we can go before becoming thirsty, but in my case it was cold, very humid (about 80% humidity) and i was resting intently, thus aided in every way by each of these factors.

I strongly believe that thirst is an excellent guide.  In my opinion it is safe in a resting fast to resist mild thirst, it may or may not be safe to resist moderate and persisting thirst and it becomes unsafe to resist heavy thirst.  If you are only mildly thirsty at 4 days i truly believe that you are safe to go for 5... but if you are extremely thirsty at 2 days, it is unsafe to pursue 3. 

If you were comfortable at 4 or 5 days dry (only mildly thirsty if thirsty at all), insomnia and/or depression if they did arise would likely be the result of your deep fasting and not dehydration.  Indeed dry fasting is deeper and more intense than "water" fasting.

Also, if at 5 days you reach the point of heavy and persisting thirst then you drink a pint of water, i highly doubt that you would be able to go a additional 5 days dry since your reserves will already have become very low and would take more time and water to re-establish themselves. 

So in summary, for some it is completely safe to dry fast for 7 days or longer and for others including me, even 2 days may be too much.