How is the proposed shelf life supposed to be calculated? Will 6 months of accelerated data equal 24 months at long-term?

ICH Q1E principles will help in the calculation of shelf life. Data from the
three ANDA submission batches (i.e., 6 months), accelerated data meeting all
criteria (without significant change per ICH Q1A(R2)), and 12 months long term
data without variability will not need statistical evaluation, and with
appropriate post approval stability commitments, can be used to support
extrapolation to a 24 months shelf life.
If there is a significant change in the accelerated data, ICH Q1E, Appendix A,
provides more details regarding when intermediate condition stability data are
recommended.