Related Clues:

• Stretch of time
• It goes on and on and on
• Immeasurably long period of time
• Lengthy stretch
• Billion years
• Geologic time period
• Eternity (Var.)
• Phanerozoic, currently
• Big time for crosswords?
• Two or more eras, in geology
• Endless time
• Interminably long time
• Countless years on end
• Tons of time
• Megayears