Sorry I'm in a riddle'y mood!

The Computation



FOR my first twenty years, since yesterday,
    I scarce believed thou couldst be gone away ;
For forty more I fed on favours past,
    And forty on hopes that thou wouldst they might last ;
Tears drown'd one hundred, and sighs blew out two ;
    A thousand, I did neither think nor do,
Or not divide, all being one thought of you ;
    Or in a thousand more, forgot that too.
Yet call not this long life ; but think that I
Am, by being dead, immortal ; can ghosts die ?

What does it equal? (No answering AP Lit students!)