[Yves S L]

Hello Max! My first approximation to a technical answer to that question: long enough to set up and deliver some manner of turn-shift-transition, then the emotional impact would be mostly coloured by the emotional quality of the turn-shift-transition. If the poem is answering another poem, then it might only need to deliver a turn relative to expectations another poem has already set up, which might allow it to be even shorter and still be self-sufficient. I can give examples, or even analyze the ones given, but I trust that folk here have little trouble finding turns-shifts-transitions in poems.