Depolarizing in polarizing mainting fiber?

Does anybody know how long the light will stay in one of the two
polarization states of a PM fiber? Assuming perfect incoupling along
one of the two axis of the fiber, i would imagine that the light does
not stay in that state for infinite long fibers due to fiber
imperfections/scattering/whatever. Does anybody have a typical length
for this, or can this effect be ignored with respect to normal
attenuation? (For my application i would need PM fibers of up to 1 km
@ 633 nm)

Thanks in advance,
Bas