Related Clues:

• It keeps going and going ...
• Many many years
• A million millennia
• Units of time
• Untold millennia
• Geological time periods
• The Phanerozoic, e.g.
• Near-infinite timespans
• Lengthy units of time
• See 41 Down
• Era after era
• It's split into eras
• Incalculable amounts of time
• Stellar periods
• Long, long periods