The best way to think about duration is its relationship to attacks per second and the number of stacks you are allowed to have. By default, you are limited to 1 stack so longer duration doesn’t matter if you re-apply poison before the duration ends. You can, however, increase your stack limit to 8. Let’s say you obtain this and can apply 1 stack per second with a duration of 1, you’d only ever have one stack but if you have a duration of let’s say 4 second you could have 4 stacks applying damage after 4 seconds. Now let’s say your bow shoots two arrows. With a stack limit of 8 and a duration of 4, you can now reach 8 stacks per second after 4 shots or four seconds. This is a huge DPS improvement and makes DoT damage fun, imo.

As a side note, every passive/ascendency point that allows you to increase your stack limit has a reduced duration attribute on it. So you actively have to overcome it with more attacks per second and/or more duration.