1) Ages 2) Ages and ages 3) Almost forever 4) Bajillions of years 5) Big stretches 6) Big times 7) Billion-year spans 8) Billions of years 9) Blue moons 10) Centuries upon centuries 11) Earth science chapters 12) Epochs unlimited 13) Eternities 14) Forever and ever 15) Forever, it seems 16) Forever, or close to it 
periods of time of such duration as to be incomprehensible; also celestial beings.