  We investigate the lifetime of macroscopic entanglement under the influence
of decoherence. For GHZ-type superposition states we find that the lifetime
decreases with the size of the system (i.e. the number of independent degrees
of freedom) and the effective number of subsystems that remain entangled
decreases with time. For a class of other states (e.g. cluster states),
however, we show that the lifetime of entanglement is independent of the size
of the system.
