"
How long will I love you?
As long as stars are above you,
And longer if I can
Ellie is trying to say is that:
As long as the stars are above you: till the end of your days, implied by the next vers "And longer if I can", meaning that even he/she will not be in her /his life, she/he will love him/she as long as he/she will can.
@Kungfu, eternity is at the end of all, can`t be more then eternity.