When subjected to long-term aging at intermediate temperatures, the 15-5PH precipitation hardening martensitic stainless steel is subject to a complex evolution of its microstructure that impacts its service mechanical properties. This evolution includes possible evolution of minor austenite phase, of the copper-rich precipitates, as well as unmixing of the chromium solid solution and formation of silicon-rich precipitates. In this work, a systematic quantification of all these evolutions as a function of aging temperature and time is obtained by combining advanced characterization tools, notably phase and orientation mapping in the transmission electron microscope, atom probe tomography, and small angle X-ray and neutron scattering. Results show a remarkable stability of austenite and Cu precipitation, and evidence the kinetics of Cr unmixing and Si-rich phase formation. We proposed a phenomenological model for the Cr composition fluctuations' amplitude and characteristic length increase with aging and put it in relation to hardness' evolution.