We performed laboratory experiments of thermochemical convection in order to determine the rate at which an initially density-stratified, and hence layered, mantle will be homogenized. In the experiments, two layers of fluid with similar viscosities are superposed and heated from below. Initially, in the stratified regime, mechanical entrainment into both layers occurs by viscous coupling and theoretical models for the entrainment rate agree well with our experimental data. Over time entrainment and mixing reduce the density difference between the layers until the doming regime is reached. The density interface becomes unstable, thermal plumes of the dense fluid penetrate into the overlying layer and the entrainment rate reaches a constant. Applying our results to the Earth's mantle, we confirm that it is possible for a compositionally distinct layer to persist over the age of the Earth, even if the initial density difference is less than 2%.