There's a ton of extra info we would need, I'm afraid. In my map, regular units have ~500 health and heroes can have anywhere from 500-3000 health, depending on level, items, and how they allocate their stat points. Health regeneration and regular damage are quite high, though, so I'd probably want the poison to be dealing at least 20 damage per second at level 1.
However, in the normal, vanilla game, 20 damage per second poison would absolutely crush everything. Maybe not to the point of just steam rolling, but I think it would be so powerful that it would ruin the game.
Just note that this ability is an orb effect (or maybe buff placer, I'm not sure), so you cannot use something like mask of death with this ability. This ability also does not stack the way some people think. It stacks perfectly if two units with it hit the same unit, but if one unit hits a unit with it multiple times, it still only has 1 stack. Basically, the number of stacks is the number of different units with the ability that has hit them.