Here's why:
* Hyperbole: An exaggeration used for emphasis or effect.
* Idiom: A phrase with a figurative meaning that is different from the literal meaning of the words.
"Half of forever" is an exaggeration. It's clearly not meant to be taken literally, as "forever" is an infinite concept. The phrase is used to convey the idea of a very long time, perhaps a lifetime or a very long relationship.
Let me know if you have any other phrases you'd like to analyze!