I would start once out of bounds in your scenario.

In response to BillyMac's scenario, my philosophy is that the ball is at the player's disposal when a member of that team could possess it and get outside the boundary in a reasonable manner. That doesn't mean they have to sprint, or even run, but they also can't delay.

If they do walk slowly, or otherwise delay, I'll make a judgment on when they should have reached the boundary and start my count then.

Here's a query:

Made basket by A in transition. B1 and B2 are setting up about the FT line extended waiting for B3 to come from the BC to make the throwin. Do you wait for B3 to come all the way, or do you start early?

Total judgement call once again...if in the second quarter I see B3 running from the BC obviously going to handle the throw-in while his teammates set up for a press break I am probably going to allow him reasonable time to get to the ball before I start my count..and if I allow it in the second quarter I am allowing it in the fourth quarter...great EOG strategy for a coach to use, especially if you have TO's left...worst case if official begins his count you use a TO to keep the ball...