As much as you can. Since bunning is one popular way to protective style of course the longer your hair is, the more of it that is used to make the bun. I don't think it's that serious that you have to measure inches. The idea is to keep your hair from drying however you can do it and away from your clothes. The more of it you keep hidden from the drying hair, the "safer" it is from the elements. The ends just get more attention coz they're the oldest and therefore most in need of extra TLC. But they are attached to the rest of your hair so you can't protect them w/o protecting a good length of your hair, especially if it's long.
And I don't think it matters whether hair is natural or relaxed or texlaxed. All hair benefits from protective styling.