Means to have been a long time ago.
Commonly said by Green Day in the song "Whatsername" in which the lyrics "forever ago" are repeated several times.
It can also be said by stupid retards without good grammar, people too lazy to express themselves in other words, and by people who really DO feel it's been forever ago.
Person 1: Hey, has that girl you met at the DDR party called back you yet?
Person 2: Nah.... it's been forever ago; dude, she isn't calling.