Possible Clues:

• The 60's or 70's, e.g
• Two-and-a-half presidential terms
• Went off, it's said, over a number of years
• Decennium
• The '70s, e.g.
• Wasted away, we hear, over a number of years
• The '80s, say
• It takes years for this to have rotted, by the sound of it
• Period just short of 26 years? Sounds bad
• Group of ten became rotten, we hear