Poison in the real world just doesn't work the way the rules say it does. If you get bit by a rattlesnake, you'll be getting worse and worse over the course of hours, not just a minute.
The rules I always wanted to write, but could never get simple enough, worked like this:
Every poison has a damage rating, a damage cap, a speed, a duration, and a DC. They might also have special rules.
For instance - Arsenic. Dmg - 1d6 Con (DC 12, slow), Dmg Cap - 12 Con.
Blue Ring Octopus poison. Dmg - 2d6 Dex (DC 20, fast), Dmg Cap - 20 Dex/Con. Special: Once the creature's Dex is reduced to 0, additional ability damage is converted to Con damage.
Drow sleep poison - Dmg - 1d6 Str (DC 17, medium), Dmg Cap - 15 Str.
The speed ratings are slow, medium, and fast.
Slow poison calls for a save one minute after the poison is introduced into the creature's body. Each minute thereafter the creature makes another save. After 10 minutes, the saves slow to one every ten minutes, then after an hour they slow to one every hour. After a day, or once the poison reaches its damage cap, the creature needs to make no more saves.
Medium poison calls for a save immediately, then another one a minute later. Each minute thereafter another save is required, until a day passes or the poison reaches its damage cap.
Fast poison calls for a save every round, until a day passes or the poison reaches its damage cap.
A special rule for poisons would say that nat 1s don't automatically fail a save against poison. Some creatures are just too tough to be affected by some poisons.