5 x Poison Level is the damage for a failed save. Successful save takes half. Note, a successful save will never take a PC below 0 HP. At worst, they will be at Death's Door.
At the most basic, a poison would be rated by levels. Most traps would be within a level or two of the dungeon level, so most traps in a 3rd level dungeon would be rated at third level.
Most creatures would have poison rated at their HD, so a 5 HP poisonous snake would be rated level 5. Depending on the creature, the poison may be a higher level, and / or have a bonus or penalty to the save.
For example: A Giant Centipede would be rated Level 2 for poison, but the save would be made at + 4
I would also rule the damage takes place at a rate of 10 HP per minute (real time) until it's run it's course. I enjoy the antics my players come up with to try and save their collective asses, and the time limit built in would lead to one hell of a challenge. Do not tell the players how much time they have - let them sweat. If you want to allow a rerolled save if someone wants to suck the poison out, some other bizarre actions, go for it. At the very least, let them make the effort even if it will be fruitless (I would allow a roll of natural 20 to work in nearly all cases, but it doesn't give back the damage that has already taken place - it just prevents further damage;)This all assumes the poison is from a bite, prick, slash, etc.
Contact, airborne and ingestive are other ways that PCs can be poisoned.
For contact poison, i'd rule that wearing gloves protects the PC, but if they don't know they touched a poisoned surface, those gloves may be deadly to them or others minutes or hours later. Fun fun ;)
For airborne, you could go with damage, or treat it somewhat like a Stinking Cloud spell, except that removing yourself from the area will not immediately lessen the effects.
Ingestive poison could do damage, incapacitate or cause retching and the like (think food poisoning).
Damn! After thinking about all these fine uses and the role playing potential, my players may need to watch out this Saturday night. Then again, they are exploring Rappan Athuk ;)