An accurate knowledge of the parameters governing the kinetics of block copolymer self‐assembly is crucial to model the time‐ and temperature‐dependent evolution of pattern formation during annealing as well as to predict the most efficient conditions for the formation of defect‐free patterns. Here, the self‐assembly kinetics of a lamellar PS‐b‐PMMA block copolymer under both isothermal and non‐isothermal annealing conditions are investigated by combining grazing‐incidence small‐angle X‐ray scattering (GISAXS) experiments with a novel modelling methodology that accounts for the annealing history of the block copolymer film before it reaches the isothermal regime. Such a model allows conventional studies in isothermal annealing conditions to be extended to the more realistic case of non‐isothermal annealing and prediction of the accuracy in the determination of the relevant parameters, namely the correlation length and the growth exponent, which define the kinetics of the self‐assembly.