I'm a lot like @darko...I've never sold my games back, ever, so I had a pretty substantial collection before I "collected". I was a Nintendo kid...had the ColecoVision when I was real little (my parents bought it just before I was born, guessing my age should be easy...) and then the NES, SNES, N64, Virtual Boy, GameCube, and every type of GameBoy. Then I met my husband, and he was a Sega kid, and he had sold very little of his stuff.
When we started seriously dating, he gave me a Dreamcast with a necklace in the CD tray...it was true love. Since then, we've collected games as a hobby we can do together. It's actually both good and bad, as our collection is huge because of it. He plays a lot of shooters, simulations and horror games, I play mostly RPGs, puzzle games, and point 'n click stuff. It really means that a big release doesn't go by without one of us wanting the game...and in the case of Star Wars: The Old Republic, both of us.
To answer your actual question: The two of us have been collecting together for about 8-9 years now.