**The batteries should weigh roughly the same as everything else on the airframe combined.**

With a given aircraft -- everything known except for the mass of the batteries -- the absolute maximum flight time occurs when the weight of the batteries is twice as much as the weight of everything else on the airframe combined.
However, many other characteristics -- time to get up to altitude, responsiveness, avoiding stall, etc. -- work better with less battery weight, so a better rule of thumb is batteries roughly the same (one times) the weight of everything else combined.
Reference: Andres 2011. [Optimal Battery Capacity][1]. via [Heino R. Pull][2].

This rule of thumb applies to all kinds of battery-powered aircraft -- fixed-wing, quadcopters, helicopters etc.

(Forgive me for recycling my answer to https://robotics.stackexchange.com/questions/262/effectiveness-of-a-mobile-robot-in-relation-to-mass/305#305 ).

EDIT: fixed a self-contradictory sentence -- thanks, Gustav.

  [1]: http://api.ning.com/files/aFMLPsgnHmZXdNMAY8PM0KekqE4uAQSMx2C*usF6KDpKiqpO4L9-hrUKYBsS-xtRMqo-K4GR5TPsM2Isq56oMw__/battery_capacity.pdf
  [2]: http://www.heino.com/?page_id=24443