  Measured hard photon multiplicities from second-chance nucleon-nucleon
collisions are used in combination with a kinetic thermal model, to estimate
the break-up times of excited nuclear systems produced in nucleus-nucleus
reactions at intermediate energies. The obtained nuclear break-up time for the
$^{129}${Xe} + $^{nat}${Sn} reaction at 50{\it A} MeV is $\Delta$$\tau$
$\approx$ 100 -- 300 fm/$c$ for all reaction centralities. The lifetime of the
radiating sources produced in seven other different heavy-ion reactions studied
by the TAPS experiment are consistent with $\Delta$$\tau$ $\approx$ 100 fm/$c$,
such relatively long thermal photon emission times do not support the
interpretation of nuclear breakup as due to a fast spinodal process for the
heavy nuclear systems studied.
