Also good points. How about this: If a victim is able to endure the effects of the poison for a full minute then the effects cease.
That's probably a nice compromise. A rare occurrence*, but a solid compromise.
[sblock=* How Rare?]
Assume a Dwarf Fighter 5, with a 16 Con. Fort Save is +7 / +9 vs. Poison. Hit with Arsenic, always rolls a 9 on his save. DC 13.
Round 1, make his save, lose one Con (15). Save now +8 v Poison.
Round 2, make his save, lose one Con (14).
Round 3, make his save, lose one Con (13). Save now +7 v Poison.
Round 4, make his save, lose one Con (12).
Round 5, make his save, lose one Con (11). Save now +5 v Poison.
Round 6, make his save, lose one Con (10).
Round 7, make his save, lose one Con (9). Save now +4 v Poison.
Round 8, make his save, lose one Con (8).
Round 9, make his save, lose one Con (7). Save now +3 v Poison.
Round 10, fail his save, lose 1d4+1 Con (5 to 2). Save now +2 or +1 v Poison. Poison ends.
Assume a Human Fighter 5, with same base stats. 14 Con, +6 vs. Poison. Hit with Arsenic, always rolls a 9 on his save. DC 13.
Round 1, make his save, lose one Con (13). Save now +5 v Poison.
Round 2, make his save, lose one Con (12).
Round 3, make his save, lose one Con (11). Save now +4 v Poison.
Round 4, make his save, lose one Con (10).
Round 5, make his save, lose one Con (9). Save now +3 v Poison.
Round 6, fail his save, lose 1d4+1 Con (7 to 4). Save now +2 to +1 v Poison.
Round 7, fail his save, lose 1d4+1 Con (5 to -). Save now +1 v Poison, or Dead.
Round 8, fail his save, lose 1d4+1 Con (3 to -). Save now +0 v Poison, or Dead.
Round 9, fail his save, lose 1d4+1 Con (1 to -). Save now -1 v Poison, or Dead.
Round 10, fail his save, lose 1d4+1 Con (-). Dead
The Dwarf is a "best realistic case", the Human is a baseline hero case. The Dwarf barely survived (hooray racial bonus against poisons), and will need 11 to 14 days to recover (or up to 7 Lesser Restoration spells). The human died as early as round 7 (without rolling an early failure), and was guaranteed to die by round 10.
Congratulations, by standard saving throw rules, you've got a very deadly set of poison rules. I hope it works out well for you and yours. Good luck.[/sblock]