Maybe it's the opposite. Maybe he is becoming more powerful and thus needs to feed more. It would still probably take billions more years either way for him to need to feed that much.

"He's been needing to feed much more often as a side effect of the Annihilation storyline. Annihilus and Thanos held him captive for the better part of a year, feeding him very little. "

"@Satyrquaze said:

"He's been needing to feed much more often as a side effect of the Annihilation storyline. Annihilus and Thanos held him captive for the better part of a year, feeding him very little. "

no he has been steadly needing more to feed before that happend... I think he is running out of power in a sense because he is never at full power or fully satisfied anymore... "

I have a different theory: Galactus serves as a balance between Eternity and Death. When he just woke up, the universe was underpopulated, with only a few species and planets alive, and very few souls were needed to maintain the balance between life and death. As life spread throughout the universe, more deaths were needed to maintain the balance, and thus Galactus became hungrier in response.
 
A similar theory is that, as time progressed, Abraxas, the anti-thesis of Eternity, became more and more powerful, and thus Galactus was forced to expend more energy in order to keep him at bay.

" I have a different theory: Galactus serves as a balance between Eternity and Death. When he just woke up, the universe was underpopulated, with only a few species and planets alive, and very few souls were needed to maintain the balance between life and death. As life spread throughout the universe, more deaths were needed to maintain the balance, and thus Galactus became hungrier in response.  A similar theory is that, as time progressed, Abraxas, the anti-thesis of Eternity, became more and more powerful, and thus Galactus was forced to expend more energy in order to keep him at bay. "

" e more and more powerful, and thus Galactus was forced to expend more energy in order to keep him at bay. "Theory, or not, it's still a very rational and well conceived one. "