In other words, until eternity.

I'm yours.

And I'll be yours 

until the stars fall from the sky.

Yours until the rivers all run dry.

In other words, until I die.

I'll be yours 

until the sun no longer shines.

Yours until the poets run out of rhyme.

In other words, until the end of time.

And I'll be yours 

until two and two is three.

Yours until the mountains crumble to the sea.