If we are talking about as collectors, picking up whatever stuff we need for collection, even if we played it before back in the day...Most of my collection perhaps? 60% I would say. Like I don't have a working Genesis or NES right now, maybe a dozen or so games there, I have an assortment of cheap PS3/360 stuff I grabbed, PS2/PS1 games I use to play that I wanted to have, but haven't sat down to play a lot of these games at all. I'm generally fairly occupied by newer stuff I get, so alot of what I buy is for pure collection purposes. It seems like lately, the only time I go to play retro stuff, it's at gamer meetups I've been going too such as beating Turtles in Time SNES for the first time last month and playing a rom of Little Samson last weekend.
Of games I just have never played before in any way, that stuff is much lower, maybe 20 to 30% I think. At the moment my collection is generally more about collecting stuff I use to like or remember playing, so I don't keep alot of assorted random titles I might have grabbed at the Flea Market or Thrift Shop. I have some new stuff I haven't played yet like Twisted Metal and Ni No Kuni on PS3 along with the Ninja Gaiden game and Pikmin 3 on Wii U.