The vertical speed with which Samus moves is controlled by how high or how low each bomb is placed relative to the previous one.
The speed of the IBJ ranges from moving fairly fast with high bombs, to no movement or even losing height with low bomb placements.
This tech alone only requires maintaining an IBJ for a vertical distance of approximately 11 tiles (the same distance as Samus can jump with HiJump),
however, it may require doing this multiple times consecutively (i.e. if there is a ledge to land on mid-way).
See also: Tech Page