So, possibilities, every hour you don't meet the survival conditions you:
- Can't heal any wounds you have (thanks Roger)
- Can't rest or regain spells
- Take 1 hit point of damage
- Take one hit die of damage (what, you roll your hit die and subtract?)
- Lose some fraction of your hit points (1/4, 1/2) [but that would mean the distress affects you less and less]
- Save or die
- Save or one of the other possibilities here happens
- Save against your Con score or die
- Save against your Con score or one of the other possibilities here happens (thanks Zavi)
- Any of the other possibilities here but with accumulating negative modifiers
- Get negative modifier to all dice rolls (combats, other saves)
- Take damage multiplied by your level (this is related to 4 but simpler to do, again, thanks Roger)
- Become unconscious (this is built into my game when characters reach 0 hp, so a little redundant)
I think it would be easy enough to remember that 1 and 2 apply any time you are distressed but I could do without them for simplicity's sake. I think 6 and 8 are too abrupt to allow for much drama to build in the wilderness. For many of these, higher level characters will be much better off but not for 4, 5, 8, 9, or 12.
I'm drawn to the Constitution save because it makes sense that tougher characters could survive better. I'm also drawn to saves in general because every hour could be a little drama where players see if they will fall unconscious or not. And it could be interesting to have the toughest characters trying to save the rest of the party by dragging them into a shelter (or Tauntaun).
So I'm leaning toward:
Each hour you're in distress, roll under your Constitution or take 1 point of damage per level.
Once you have that simple base you can add some layers on top. For example, for the freezing water I thought about above, keep the same rule but make it each round you are immersed.