Related Clues:

• A looooong time
• Forever, facetiously
• 1 billion years
• 365 billion days, astronomically
• Interminable stretch
• Almost eternity
• Longish span of time
• Millennia on end
• Geological span
• See 108 Across
• History unit
• Loooong time
• It's comprised of ages and ages
• Heck of a long time
• Really big span