Different people react differently to deprivation. With water, there's not much variance, and after 2 to 3 days depending on local conditions, you'll shut down, go into a coma, and die. There's not enough water for your organs, blood, brain etc to function.
With starvation, going a month without dying is not uncommon for peopel that do hunger strikes, or people in real starvation conditions. Physically fit people can go a week, easy, with pretty much no ill effects- my martial arts school requires a fast before you test for black belt ranks, and I have done 5 days without food, felt great, and gone in and performed & fought without much problem. Your endurance suffers a little.
For food, I'd allow your Con bonus, in days, without any checks (minimum 0, obviously), then a fort save a day, DC 5 and incrementing by 1 every day. If you fail a fort save, you take a point of Con damage and are considered fatigued. You can't regain the Con naturally until you start eating agin. You don't lose the fatigued condition until you somehow get your Con back up to normal, either by eating or restoration-type magic.
You continue to make saves every day, taking another point of Con damage every time you fail; once your Con drop to half your normal value, you are considered exhausted instead of fatigued. Again, you keep this until you get your Con back to normal somehow.
When your Con drops to 0, you die.
So an average guy, Con 10, could probably make the first 5 or 6 days of save, no problem; after the first couple of days, he won't even feel hungry any more, and as long as he gets enough water, he's in reasonably good shape.
Somewhere around a week in, he's /fatigued/, he's noticeably weaker and his coordination is a little off. He'll probably make 1/3 of the saves or so from here on out, so he might make it another 15 days, getting gradually weaker, until he is unable to go on.
