I know it is a tricky question. What do you think, how many Bitcoins are lost forever and are 'out of the network' by lost private keys?
i found this on reddit,may this help to answer your question :
You could do an analysis of how often coins move between wallets, and use that data to determine which coins are unlikely to move again. Like, if 90% of coins move at least once every 6 months, you could conclude that the ones that haven't moved in 12 months probably never will. Oversimplification, but it could probably be done with a reasonable amount of certainty - certainly, some coins are just in cold storage, but I imagine that would only account for a small portion of your sample set. Good enough for a broad overview of the market. KallistiTMP
