When you are comfortable with your profit or better still, when you have decided from get go the target price for your exit from a particular trade, that when you should sell. However, there are market conditions that could make you sell earlier or later than planned.
I will not even decide after watching the profit. I think i will decide by analysing the volume if it keeps on increasing then i will probably hold. if the graph and anyone can see that it be is experienced enough, the graph goes to a high and then go down a little bit and then again go to that high again, this indicates the resistance and if enough funds get invested in it then the graph goes up, but it is rare that the investors will follow the increasing bitcoin price and the bitcoin price falls due to lack of investors investing in it, in the end it all depends on your ability to read the graphs and then decide whats the next to do.