We decide what's the bottom. I started buying today @3500.
Personally I will buy the way up and the way down.
So both at 3000 and 4000 I will buy a small amount.
We will invest if we already saw the bottom for us even if the market is keeping on dumping. Its all depend with our strategies but if you still want to know when the market down trend ends, I think it cannot be answered by anyone. Keep on investing at no specific price level, just have a good volume to hold.