We present quantitative experimental measurements of an inverse energy cascade in deep rotating turbulence. Experiments were performed in a rotating cylindrical water tank configuration with energy injection at a large wavenumber, k_i. For k < k_i the steady state energy spectrum is very well described by E_k=C ϵ^-2/3 k^-5/3, the 2D energy spectrum. The temporal evolution of energy spectra towards steady states was also measured. We identify a “front”, located at k^*(t), which propagates from large to small wave numbers, behind which the spectrum attains its steady state value. For k < k^*(t) the energy is considerably lower than the steady state. The propagation of these fronts was measured for different energy injection rates and rotation frequencies. These measurements are in good quantitative agreement with the 2D turbulence, which predicts (k^*(t))^-2/3 - (k_i)^-2/3 ∝ ϵ^1/3 t.