If you're big enough to move the market with your own trades, then you know exactly when to sell, which is right after you've pumped up the price.  And I am using the word "pumped" here correctly.

Other than that, what Gloober said.  No one really knows, because no one knows the future.  And in bitcoin, there isn't exactly inside info anyone can trade on that I know of.  It's all guesswork.  For instance, the price has risen pretty steadily to $1250.  Wouldn't you think now would be a good time to sell?  Maybe--but you don't know where we're going from here.  We could crash or we could go to fucking Pluto.  No one knows, not me, not you, not Donald Trump, not the bum on the street corner.