In survivalist culture, most people know about the Rule of Threes: You can die in three hours from exposure to the elements, three days without water, and three weeks without food.*
There are two caveats I'd like to mention in connection with this Rule.
Number 1 is to keep in mind that these are general statements and will vary (sometimes greatly!) depending upon the environmental conditions and the person in the situation. This Rule is really meant as a guide to prioritize your energy expenditures in, and preparation for, a survival situation.
Number 2, remember that the most important survival tool is your ability to think. Realizing this, you shouldn't fool yourself into believing that you have three whole days before you experience any difficulties without water. Even minor dehydration can impair decision-making and analytical thinking.
The same goes for eating. Try skipping a meal or two and see how you do when trying to solve problems. Keeping your blood sugar up could save your life just as easily as could building a shelter, so you shouldn't skip easy opportunities for food and water if they present themselves.
Despite these points, let's reaffirm once again that the Rule of Threes is very useful as a prioritizing tool. Without being familiar with this Rule, a person may respond to the need which calls out the strongest (typically hunger) and spend time chasing rabbits, while delaying creating a shelter or fire.
On the other hand, someone familiar with this Rule may focus on protection from extremes in heat and cold, which can 1) prevent dehydration and 2) help your body to regulate its temperature, which conserves energy. Thus, having a shelter can lower the need for water and caloric intake, and it is far more efficient to conserve your body's water and energy supply than to replace it. The Rule of Threes stands vindicated!
Just one last point: You may have noticed that a few other important items are lacking from this list: First Aid, Signaling, and Hygiene. The reason for this is more than likely because these actions may have a varying priority in different circumstances. Obviously, if someone is bleeding profusely, even shelter takes a back seat until the severe bleeding is under control; but scrapes and cuts can be taken care of after the more immediate needs are met. Likewise, if you know that there are people in the vicinity, blowing a whistle to help them find you is more important than gathering berries. Lastly, while hygiene is important, it is typically not as pressing as the other matters.
All in all, the Rule of Threes can serve you well in organizing your preparation for and behavior in a survival situation, as long as you think about your individual situation instead of doggedly following the list.
* I have also heard this called the Rule of Fours, with the time limits obviously increased; either system may be more accurate with different people or situations; however, I prefer to assume the worst. I have also heard of people adding a "X minutes without air/first aid" and "X monthes without hope". "Air" seems to be unnecessary to mention because 1) this is a rare situation and 2) people generally react immediately if they can't breath; there's less of a reason to tell them that they need to make it a priority;). The hope aspects seem problematic because it seems the time limit will vary greatly depending upon the person. Some people may die in a less than a week without hope, because they won't bother even trying to stay alive.