In the Marvin Gaye song "I Heard It Through the Grapevine" the refrain is:
Ooo I heard it through the grapevine
Not much longer would you be mine
Ooo I heard it through the grapevine
And I'm just about to lose my mind
Why not "will" instead of "would", because in this song the character is still going out with that girl? They have not broken up yet even if he thinks that it will be the end soon.