Related Clues:

• Stretch of time
• Immeasurably long period of time
• Lengthy stretch
• Billion years
• Geologic time period
• Phanerozoic, currently
• Big time for crosswords?
• Two or more eras, in geology
• Interminably long time
• Countless years on end
• Tons of time
• Megayears
• What a houseguest's stay might feel like
• Astronomer's time period
• Long time period