An in-depth case study of precipitation kinetics for alloys is presented utilizing recent progress in high-precision isothermal dilatometry by measuring relative length changes down to the range of 10-5 and covering large timescales exceeding 105s. Using a dilute Al-Mg-Si alloy as model system, the different phases that form during isothermal heat treatment could quantitatively be analyzed both with respect to the absolute amount of precipitates and with respect to the underlying kinetics. Owing to the distinct length change features upon multi-step precipitation processes, the formation of the metastable - and -phases can unambigously be detected and furthermore can specifically be distinguished and resolved. From the reaction rate analysis of the precipitation-induced relative length change, that was isothermally measured for temperatures between 170 degrees C and 260 degrees C, the evolution with time of the atomic fraction of both the - and the -phase was determined. The results were also used to construct the isothermal time-temperature-precipitation diagrams which are important for technologically relevant processes.