  We consider photonic tunnelling through evanescent regions and obtain general
analytic expressions for the transit (phase) time $\tau$ (in the opaque barrier
limit) in order to study the recently proposed ``universality'' property
according to which $\tau$ is given by the reciprocal of the photon frequency.
We consider different physical phenomena (corresponding to performed
experiments) and show that such a property is only an approximation. In
particular we find that the ``correction'' factor is a constant term for total
internal reflection and quarter-wave photonic bandgap, while it is
frequency-dependent in the case of undersized waveguide and distributed Bragg
reflector. The comparison of our predictions with the experimental results
shows quite a good agreement with observations and reveals the range of
applicability of the approximated ``universality'' property.
