I have about 1TB stored (I need to get another external... badly)... and have watched about 95% on it.
In terms of how much I have watched...
Series: 214, OVA: 66, Movies: 75, Spcl.: 21, Eps: 4556 (+ 553 of airing/unfinished series)
I delete all the garbage that I watch and I don't archive the neverending shit either (One Piece etc). Also, whenever I purchase a series, I clear it off immediately to make more room.