Read an article once, author claimed up to 30% coins are lost forever.
I wouldn't go that much or believe that article like most of them, but there is definitely quite a lot of coins missing/lost would you count satoshi's million+ as lost as they have never been touched for years? I am guessing anywhere from 10-15% but who knows and in the future there will obviously be a hole lot more being lost which is good for everyone else who doesn't lose them i guess them, smaller supply higher value.
I think it would be hard to find amount. You might be able to find BTC adress with no activity, but part I don't see is how you could tell cold storage holding (with no one depositing more) compared to a lost account.
I do think it is not something that will repeat its self. I would guess most lost were CPU and GPU days. But that is just my feeling as price was lot lower then.