  Using the fact that we only observe those modes which exit the Hubble horizon
during inflation, one can calculate the entanglement entropy of such
long-wavelength perturbations by tracing out unobservable sub-Hubble
fluctuations they are coupled to. On requiring that this perturbative
entanglement entropy, which increases with time, obey the covariant entropy
bound for an accelerating background, we find an upper bound on the duration of
inflation. This presents a new perspective on the (meta-)stability of de Sitter
spacetime and an associated lifetime for it.
