Context : The visibility time of planetary nebulae ( PNe ) in stellar systems is an essential quantity for estimating the size of a PN population in the context of general population studies .
For instance , it enters directly into the PN death rate determination .

Aims : The basic ingredient for determining visibility times is the typical nebular expansion velocity , as a suited average over all PN sizes of a PN population within a certain volume or stellar system .
The true expansion speed of the outer nebular edge of a PN is , however , not accessible by spectroscopy – a difficulty that we surmount by radiation-hydrodynamics modelling .

Methods : We first discuss the definition of the PN radius and possible differences between the observable PN radius and its physical counterpart , the position of the leading shock of the nebular shell .
We also compare the H \alpha surface-brightness evolution predicted by our radiation-hydrodynamics models with the recent H \alpha surface-brightness radius calibration of \citet [ ] [ PhD thesis , Macquarie University , Australia ] frew.08 and find excellent agreement .
We then carefully investigate the existing spectroscopic data on nebular expansion velocities for a local PN sample with objects up to a distance of 2 kpc with well-defined round/elliptical shapes .
We evaluate , by means of our radiation-hydrodynamics models , how these observed expansion velocities must be corrected in order to get the true expansion speed of the outer nebular edge .

Results : We find a mean true expansion velocity of 42 km s ^ { -1 } , i.e .
nearly twice as high as the commonly adopted value to date .
Accordingly , the time for a PN to expand to a radius of , say 0.9 pc , is only 21 000 \pm 5000 years .
This visibility time of a PN holds for all central star masses since a nebula does not become extinct as the central star fades .
There is , however , a dependence on metallicity in the sense that the visibility time becomes shorter for lower nebular metal content .

Conclusions : These statements on the visibility time only hold for volume-limited samples .
Extragalactic samples that contain spatially unresolved nebulae are flux limited , and in this case the visibility time directly depends on the limiting magnitude of the survey .
To reach a visibility time of 21 000 years , the survey must reach about 7 magnitudes below the bright cut-off of the planetary nebula luminosity function .
With the higher expansion rate of PNe derived here we determined their local death-rate density as ( 1.4 \pm 0.5 ) { \times } 10 ^ { -12 } PN pc ^ { -3 } yr ^ { -1 } , using the local PN density advocated by [ Frew ( 2008 ) ] .