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The visibility time of planetary nebulae (PNe) in stellar systems is anessential quantity for estimating the size of a PN population in the context ofgeneral population studies. For instance, it enters directly into the PN deathrate determination. The basic ingredient for determining visibility times isthe typical nebular expansion velocity, as a suited average over all PN sizesof a PN population within a certain volume or stellar system. The trueexpansion speed of the outer nebular edge of a PN is, however, not accessibleby spectroscopy -- a difficulty that we surmount by radiation-hydrodynamicsmodelling. We find a mean true expansion velocity of 42 km/s, i.e. nearly twiceas high as the commonly adopted value to date. Accordingly, the time for a PNto expand to a radius of, say 0.9 pc, is only 21000 +/- 5000 years. Thisvisibility time of a PN holds for all central star masses since a nebula doesnot become extinct as the central star fades. There is, however, a dependenceon metallicity in the sense that the visibility time becomes shorter for lowernebular metal content. With the higher expansion rate of PNe derived here wedetermined their local death-rate density as (1.4 +/- 0.5) x E-12 PN pc^{-3}yr^{-1}, using the local PN density advocated by Frew (2008).

The evolution of planetary nebulae. VIII. True expansion rates and visibility times