As the title says.
Regardless of the duration listed on the tooltip, Poison stacks (and the Primal affinity) only last to the end of the turn they're applied on, and only tick once at the end of that turn. This includes the Poisons applied by Constrict (tooltip duration: 3 turns) and Paralyzing Poison (tooltip duration: 2 turns).
Needless to say this is making most units with Primal, Poison Sting, Toxic Secretions, Constrict, or Paralyzing Poison fairly useless.