We present measurements of the energy relaxation length scale $\ell$ in two-dimensional electron gases (2DEGs). A temperature gradient is established in the 2DEG by means of a heating current, and then the elevated electron temperature $Te$ is estimated by measuring the resultant thermovoltage signal across a pair of deferentially biased bar-gates. We adapt a model by Rojek and K"{o}nig [Phys. Rev. B \textbf{90}, 115403 (2014)] to analyse the thermovoltage signal and as a result extract $\ell$, $Te$, and the power-law exponent $\alpha i$ for inelastic scattering events in the 2DEG. We show that in high-mobility 2DEGs, $\ell$ can attain macroscopic values of several hundred microns, but decreases rapidly as the carrier density $n$ is decreased. Our work demonstrates a versatile low-temperature thermometry scheme, and the results provide important insights into heat transport mechanisms in low-dimensional systems and nanostructures. These insights will be vital for practical design considerations of future nanoelectronic circuits.