Never, because in reality the basis of forex trading is through the use of leverage given by the investment banks you trade in facilitation with. Barring an unforeseen event, banks tend to limit trends by moving in the opposite position of the majority of their traders. Strong traders usually get out, because they have price sets built in. The weak ones are those who get punished, and play the game as if it were stocks.

In a time such as now where the dollar is in ultra high demand, because firms face less opportunity cost when repaying dollar debt with dollars, no amount of bank manipulation can compete with a fundamental market mechanism.