A limit order has not only a price variable but a duration variable as well. Here are the main terms related to validity duration:
GTC (Good till canceled) – an order to buy or sell a currency at a specific or limit price that lasts until the order is completed or canceled. A GTC order will not be executed until the limit price has been reached, regardless of how many days or weeks it might take.
GFD (Good for the day) – This is also called a day order, meaning that it is good only during that trading day. Orders that have been placed but not executed during the day will not automatically carry over into the next trading day. If your order is not filled during a trading session, you will have to place a new order in the next trading session. 5pm NY Time is considered to be the end of the forex trading day.
OCO (Order cancels other) – Using the same EUR/JPY example, you can place a kind of "double order" - an order to EITHER buy at 136.20 OR sell at, say, 135.10. If the price reaches 135.10, you will have an open short position while the 136.20 buy order will get canceled. If, on the other hand, the market hits 136.20, you will have an open long position, and your 135.10 sell order will get canceled.
Make sure you get a demo account with the forex brokerage of your choice and try out their trading platform. You must learn to easily handle the usual operations of buying, selling, placing limit or stop orders, changing the stop order of an open position, pulling up forex charts in various time frames, etc.