No one will tell you the exact time when it should be done. I think that buying Bitcoin is best at a time when its price has dropped significantly. Selling Bitcoin is better when its price is high. And since its price is very variable it is easy to track.
agree actually last week when we see a drastic dropped on bitcoins it was actually an opportunity to buy bitcoins on a cheap price , don't believe all FUD's that bitcoin will die just ride on it and enjoy the discounted bitcoin of price and make a good profit in the coming week or two ,very easy right ?