But you can be sure that the price of bitcoin will always rise in the long run. So if you can, buy more!
More people BUYING bitcoin = Price UP
More people SELLING bitcoin = Price DOWN
This is a simplified explanation on how bitcoin price increases and decreases, and for OP if you are asking when will the price would decrease is not a very simple question because just like tryninja said no one would really know when it will drop, but certain unwanted selling of most people are the cause of a sudden drop in price just like what happen when we had reach the price value of $2700 certain people make an emotional selling out of fear that bitcoin might drop all of a sudden then a consecutive selling of many was the reason why the price drop well hope when bitcoin increase up to $3000 we can not see this kind of drop anymore.