It is a matter of experience. Some text books say 5 seconds before. If you see a gap in the traffic you may not have 5 seconds. In any case, you have to be sure that the people around you know what you intend to do. If they don't you may become a statistic.
You should turn on your turn signal before you need to brake for the turn.
10 feet.
Signal well in advance.
mirror signal maneuver
Approximately 1 block away from the desired turn
anytime you change lanes
Maintain a safe following distance until the truck has completed maneuver
Use your signal lights.
When changing lanes you should do the following Signal check your mirrors change lanes turn signal off. What step is missing from this lane-changing procedure?
So that traffic behind you is able to anticipate your turn, and can act accordingly.
He HAS a book in his hands. You should prepare to stop AS soon AS you see the signal change.
You should use your turn signal in far enough advance so the car behind you knows you are going to turn and has enough time to slow down. However, putting the turn signal on too far in advance can be confusing because the following car will not know exactly when you are going to turn.