There are 25 frames per second
Depending on the skill it can apply its damage any number of times per second. Some of them can apply their damage more than once per frame too, if they're composed of multiple missiles that have no Next Delay.
Once poison is applied to a monster it takes damage once every frame for the entire duration it is poisoned. The skill applying the poison can apply the poison multiple times over its duration (like the Druid's Plague Avatar), but each new instance of poison overwrites the old one if its DPS is higher, and refreshes the duration.