One thing I've discovered in my own efforts at house-rule design, it's a bad idea to use a PC's Constitution or level (i.e. hit point total and/or saving throws) as a measure for how long it takes them to die. This unduly rewards characters with a high Con (especially dwarves and such), who already have a Constitution bonus expressed in their hit points. Likewise, hit points and saving throws of and in themselves are an adequate expression of how hard it is to kill a character based on their level.
In AD&D games (including 3e), where the tendency is to have characters bleed out round-by-round, I've often gone with a "countdown clock" as I mentioned in my previous post, but in order to keep its results secret from the players, what I actually do is roll 1d10 in secret once a PC reaches 0 hp, and then I reduce the number by 1 every round. If the PC is healed, he can survive (or even get up and fight again, if the healing was magical); otherwise, he'll wind up kicking the bucket at some point unknown to all the players.
In OD&D games, which are characteristically harsher about death, I prefer a more organic method. When a player drops to 0 hp, I roll 1d6 in secret (so many game mechanics in OD&D rely on that d6). On a result of 1, the lucky PC was only knocked out cold. If left alone, he'll wake up on his own after the battle. If healed, he can get up and fight again immediately. On a roll of 2 or 3, the PC is mortally wounded and in danger of bleeding out. The PC has one game turn (which in OD&D is ten in-game minutes, or 60 combat rounds, whichever comes first) to receive some medical attention, or else he dies. If the PC does get healed, he's nonetheless disabled and bedridden for several days. On a roll 4 to 6, the PC is instantly killed. This rule is also exceedingly handy to use on important NPCs and villains, since it adds a degree of randomness to "who survives and has last words" vs. "who just gets killed." And, of course, the DM can always roll the d6, ignore it, and pick a result that fits the situation.