Check out the Solaris example videos for a demonstration of using a trace to figure out which object is under the player’s crosshairs.
If there is an object you care about is under the crosshairs (which can be tested for with a cast), then record the current time and the viewed object in variables. Testing stuff with casts is also demoed into the Solaris example videos. Watch them!
If the object being pointed at currently is the stored object, and (the recorded time - the current time) > X seconds, then perform whatever behavior you want to perform. You could send an event to the object, and let the object itself handle doing whatever, for example.
An alternate way of doing this is to start a timer object within your interface blueprint whenever you are hovering over an interesting object, and stopping the timer when you are no longer hovering over an interesting object. If the timer reaches it’s conclusion, do stuff. If you use a Timeline instead of a Timer, you can use the Timeline value to drive a little progress bar (perhaps displayed near your crosshair), to give the player a visual indication of when the interaction will occur.