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It _always_ applies.  The question is: Do you care?

BCF = (1 + (extension/focal length))^2

A BCF of 2.0 means 1/2 the shutter speed or open up one stop.

Lets say 1/2 a stop is GE =>  BCF is 1.41

extension = (sqrt(BCF)- 1) * focal length

extension = 0.187 * focal length = 20%
          = 1 1/4" (30mm) on a 6" (150mm) lens

Yeah all kinds of formulas but both Calumet and Kodak made slick
calculators.  You put a target on the subject field, whip out a special
ruler measure one side of the target and the exposure factor is right there
on the ruler.

  Bellows factor is due to the inverse square law affecting
the light from the lens to the film. For B&W and probably
color negative its negligible down to subject distances of
around 4 or 5 times focal length. Bellows factor in stops is
simply the ratio of the extension at infinity focus to the
extension at the object distance of interest. For intstance,
at unity magnification the bellows extension is 2 times that
at infinity so the exposure must be increased by 2 stops or
4 times.

Just as a quick rule of thumb, when the distance from the lens to the
subject becomes less than about nine times the lens focal length you need to
start allowing more exposure.  Following this idea, for a 135mm lens, which
is a little more than 5 inches in focal length, the exposure factor starts
becoming a consideration when the lens is closer than about 48 inches to the
subject.