  Excitable media can develop spiral chaos, in which the number of spirals
changes chaotically with time. Depending on parameter values in dynamical
equations, spiral chaos may permanently persist or spontaneously arrive at a
steady state after a transient time, referred to as the lifetime. Previous
numerical studies have demonstrated that the lifetime of transient spiral chaos
increases exponentially with system size to a good approximation. In this
study, using the fact that the number of spirals obeys a Gaussian distribution,
we provide a general expression for the system size dependence of the lifetime
for large system sizes, which is indeed exponential. We confirm that the
expression is in good agreement with numerically obtained lifetimes for both
excitable and oscillatory media with parameter sets near the onset of transient
chaos. The expression we develop for the lifetime is expected to be useful for
predicting lifetimes in large systems.
