  I show that the characteristic diffusion timescale and the gamma-ray escape
timescale, of SN Ia supernova ejecta, are related with each other through the
time when the bolometric luminosity, $L_{\rm bol}$, intersects with
instantaneous radioactive decay luminosity, $L_\gamma$, for the second time
after the light-curve peak. Analytical arguments, numerical radiation-transport
calculations, and observational tests show that $L_{\rm bol}$ generally
intersects $L_\gamma$ at roughly $1.7$ times the characteristic diffusion
timescale of the ejecta. This relation implies that the gamma-ray escape
timescale is typically 2.7 times the diffusion timescale, and also implies that
the bolometric luminosity 15 days after the peak, $L_{\rm bol}(t_{15})$, must
be close to the instantaneous decay luminosity at that time, $L_\gamma
(t_{15})$. With the employed calculations and observations, the accuracy of
$L_{\rm bol}=L_\gamma$ at $t=t_{15}$ is found to be comparable to the simple
version of "Arnett's rule" ($L_{\rm bol}=L_\gamma$ at $t=t_{\rm peak}$). This
relation aids the interpretation of SN Ia supernova light curves and may also
be applicable to general hydrogen-free explosion scenarios powered by other
central engines.
