  Context. In 1998 the first accreting millisecond pulsar, SAX J1808.4-3658,
was discovered and to date 18 systems showing coherent, high frequency (> 100
Hz) pulsations in low mass X-ray binaries are known. Since their discovery,
this class of sources has shown interesting and sometimes puzzling behaviours.
In particular, apart from a few exceptions, they are all transient with very
long X-ray quiescent periods implying a quite low averaged mass accretion rate
onto the neutron star. Among these sources, XTE J0929-314 has been detected in
outburst just once in about 15 years of continuous monitoring of the X-ray sky.
Aims. We aim to demonstrate that a conservative mass transfer in this system
will result in an X-ray luminosity that is higher than the observed, long-term
averaged X-ray luminosity. Methods. Under the hypothesis of a conservative mass
transfer driven by gravitational radiation, as expected for this system given
the short orbital period of about 43.6 min and the low mass of the companion
implied by the mass function derived from timing techniques, we calculate the
expected mass transfer rate in this system and predict the long-term averaged
X-ray luminosity. This is compared with the averaged, over 15 years, X-ray flux
observed from the system, and a lower limit of the distance to the source is
inferred. Results. This distance is shown to be > 7.4 kpc in the direction of
the Galactic anticentre, implying a large height, > 1.8 kpc, of the source with
respect to the Galactic plane, placing the source in an empty region of the
Galaxy. We suggest that the inferred value of the distance is unlikely.
(abridged)
