Related Clues:
- Extremely long stretch
- Indefinitely long period
- Geologic span
- A billion years, in geology
- A billion years, to geologists
- Many generations
- Umpteen millennia
- Mind-boggling time span
- Reeeeeeally long time
- Longest division of geologicalà time
- It never ends
- Gigayear
- Forever and a day, facetiously
- It's a real stretch
- Astronomically long time