  In this paper we study the late evolution of model galaxies after a single
episode of star formation of different durations. The aim of the paper is to
discover the timescale needed to refill with cold gas the center of the galaxy.
This timescale strongly depends on the amount of gas initially present inside
the galaxy and ranges between 125 and 600 Myr. A HI hole can therefore survive
several hundred Myrs after the last SNII has exploded. If, as a consequence of
the refill of the center of the galaxy, a second episode of star formation
occurs, it pollutes the surrounding medium in a very short timescale (of the
order of 10--15 Myr), at variance with what happens if the center of the galaxy
is still occupied by hot and tenuous gas.
