You wont get a dedicated E-Mail that states your Background Check has started. All you see is the changed status in your MyGCKey. For the timeline how long it will take from that moment until you receive a decision. That can be absolutely anything. What gagan292 said that wou will get a decision within a week is highly unliekly. But you can be happy, your application is on its right way. Now you just have to wait for decision made. However long that will take no one can answer you that.