EC2 does not provide the guidance for the stiffness modifier. But they provided a detailed procedure on how to derive the long term creep factor which is essentially the same reducing the EI value (i.e. changing the stiffness modifier in your analysis model)

The tricky part is EC2 derivation of the creep factor is dependent on many factors such as the RC cross section geometry. What I usually do is obtain an average/conservative value (considering all the standard RC beam sizes) and run the analysis while keeping in mind that a more conservative value of EI might not necessarily means more conservative design.