Answer: Across all the poem
Step-by-step explanation:
I can even bet that finding the rhythm in a poem it´s a process that lasts as long as someone is reading it. BUT, have you ever read a poem to someone else? Not yourself, you will never know if you´re a good chef if only you taste your food.
Give it a shot and I assure you that something so intangible as a poem can´t be separated or classified by procedures (free poetry is a clear example).
The rhythm ends with the last period of the poem (do not confuse it with prose), but the beauty of it echoes through eternity.