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?
Its very sensitive topic and I think every newbie must have to aware before doing procedure of own bitcoin wallet on own desktop or laptop even in web wallet also. Because if you have btc on particular wallet address but if you forget private keys of your wallet then it make big trouble. As no one can use that bitcoins without private keys.