If he loved with all the powers of his puny being, he couldn’t love as much in eighty years as I could in a day.
Heathcliff measures love by intensity, not time. The hyperbole scorns a rival’s “puny” capacity and asserts his own force. Anger flares because he feels cut off from what he believes is rightfully his. This line shows how pride and passion blend into contempt. It’s powerful and also self-wounding: such extremes burn the speaker, too. Readers hear the old argument—depth versus duration—and the danger on both sides.