In the real world, when people are poisoned, the effects of the poison continue for hours, and some people linger on for days before dying or recovering. The D&D game doesn't reflect this, mainly because it's more convenient for DMs to get the poison damage over and done with and not deal with long-term bookeeping. I think that reflecting the continuing nature makes the effects of poison more realistic, and if you don't mind recording a little bit more data it's worth it.
So, here's the variant rule.
Variant Rule: Continuing Damage For Poison
In addition to the normal primary
and secondary damage, poison continues to deal damage over an extended
period of time. This continuing damage only occurs if the target fails
one or both of the poison's saving throws (a character that succeeds at
both saving throws suffers no additional damage from the poison).
The poison's effects continue for a number of hours equal to the poison DC (if both saving throws are failed, the effects continue for 2x the poison's DC instead of just the DC).
Every hour of the continuing effect, the character must attempt another saving throw against the poison's DC. Failure means the character suffers another point of ability damage of the type normally deals by the poison. If the poison deals one type of primary damage and one type of secondary damage (or deals two types of damage at the same type, as bloodroot does), the continuing damage is of the same type as the failed saving throw (primary or secondary). If the character failed both saving throws, the damage type alternates every hour (between the two types of damage) until the poison runs its full course (2x the DC in hours), starting with the primary damage (primary the first hour, secondary the second hour, primary the third hour, and so on).
Example: Small centipede poison is DC 11 and deals 1d2 Dexterity damage for its primary and secondary effect. A character that fails a single save against this poison will have to make saving throws every hour for 11 hours, suffering 1 point of Dexterity damage each time one of these saves is failed. A character that fails both saves would make these checks every hour for 22 hours (DC 11 x 2).
Poisons with a primary or secondary effect that does not deal ability damage do not deal continuing damage of that sort (the only continuing damage is ability damage). This means that poisons that deal no damage, hit point damage, cause unconsciousness, or other nonstandard effects do not require continuing saving throws. However, failing saves against the primary and secondary damage still causes the poison's continuing effects for twice the DC--but every other hour there is no effect from the poison (because that hour's damage would be from the nonstandard effect, which does not deal continuing damage).
Example: Bloodroot is DC 12 and deals "0 damage" for its primary effect and 1d4 Con and 1d3 Wis for its secondary effect. A character that fails the first save but makes the second save does not suffer any continuing damage (the "0 damage" of the primary effect does not recur). A character that makes the first save and fails the second save must save every hour for 12 hours, suffering either 1 Con or 1 Wis (alternating each time between the two types of damage dealt by the poison's secondary damage) for each continuing save failed. A character that fails the primary and secondary saves must save every hour for 24 hours (DC 12 x2), with every other hour's save made to avoid taking 1 Con or 1 Wis (from the secondary effect); every other hour would be damage from the "0 damage" primary damage of the poison, which does not recur, so the net effect is a saving throw every two hours for the poison's continuing duration (24 hours).
Delay poison suspends all of this continuing damage for the duration of the spell, just as it suspends primary and secondary damage.
Neutralize poison ends the cycle of continuing damage (the character does not need to make any more saving throws against the poison).
A character with the Heal skill can use it to aid someone's saving throws against continuing damage just as it can help with primary and secondary damage saves.
So this means that snake poison (either DC 11 or
DC 13) continues for 22 or 26 hours (if one or both saves are failed),
and has the potential to deal another 22 or 26 points of Con damage over
time. This makes snake poison likely to kill normal people (assuming a
50% save rate and dealing an additional 11 or 13 points of Con damage over
the course of the poison), but somone with a decent Heal check (even 1
rank in Heal, taking 10 each time) means the person automatically makes
the saves and can save a person from death by continuing snake poison damage.
So if you're bitten, get medical attention!