  We present the first measurements of the temporal decay rate of
one-dimensional, linear Langmuir waves excited by an ultra-short laser pulse.
Langmuir waves with relative amplitudes of approximately $6\%$ were driven by
$1.7$ J, $50$ fs laser pulses in hydrogen and deuterium plasmas of density
$n_{e0} = 8.4 \times 10^{17}$ cm$^{-3}$. The wakefield lifetimes were measured
to be $\tau^\mathrm{H_2}_\mathrm{wf} = (9\pm2)$ ps and
$\tau^\mathrm{D_2}_\mathrm{wf} = (16\pm8)$ ps respectively for hydrogen and
deuterium. The experimental results were found to be in good agreement with 2D
particle-in-cell simulations. In addition to being of fundamental interest,
these results are particularly relevant to the development of laser wakefield
accelerators (LWFAs) and wakefield acceleration schemes using multiple pulses,
such as multi-pulse laser wakefield accelerators (MP-LWFAs).
