100 feet. The law requires a clear signal that you are turning from at least one hundred feet before you go across the centerline of any road.
For instance, if there is another car following you and waiting for an opportunity to turn right ahead of your car, he should give the proper warning to drivers on the left by flashing his lights when he has passed the hundred foot mark and signaling right with his vehicle until he has made his turn. Make sure that you can see what other people who will be impacted by this change in direction are doing BEFORE executing it! Otherwise, imagine how badly something could go wrong with someone coming up behind your vehicle without such notice… hopefully they don’t have their headlights switched off or on dim mode