Grub, food, water, sustenance, it's all needed in the wilderness.
Combined Strength and Constitution Tolerance level (days)
Then every 12 hours make a Str or Con check (at +1 cumulative per check) or else... you are weakened (-1 on attack rolls and reflex saving throws cumulative per day in this state). Also now cumulative +2 on subsequent checks. Fail again... distressed (max 2 hours strenuous activity per day). Fail again.... incapacitated (no voluntary physical activity) and in danger of starving to death within hours (losing 1d6 hit points per hour)!
Max 3 days without water until bad things happen!
Females can manage an extra day! Get that - something in the rules making it advantageous to be a female character.
Other modifiers depending on type of food if available (watermelon a good one to have), heat etc
After this time weakened, then as above to the worsening states, except 1d8 hit points loss per hour when incapacitated.
How much must one eat in a day?
Human, dwarf, half-elf, half-orc = 1-2 pounds of solid nourishment per day.
Elf 3/4 this much, halfling or gnome 1/2.
In game terms...
Standard rations = 200gp encumbrance. Assume 140gp is food, 60 is bulkiness. 140gp = 14 pounds. 2 pounds of standard rations must be eaten per day for a human, meaning standard rations last 1 week.
Iron rations encumbrance is 75gp. 5 gp is bulkiness, since the food is more concentrated and occupied less space) therefore 70gp or 7 pounds remains. 1 pound of iron rations must be eaten per day for a human, meaning iron rations last 1 week.
Ok, I know the author Kim Mohan completely fudged this so that rations last one week. But it aint a bad fudge!
There is even a table on water requirements per day depending on activity and temperature; range 5-16 pints per day for a human.