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Unactivated SIMs never expire for new activations.
Unactivated SIMs never expire for SIM swaps, since the 6-month bug was fixed a few days ago.
Activated SIMs do expire if unused for 6 months, though there are various ways to avoid this, as listed in section 13.3 of the T&C. https://giffgaff.com/terms#Cancellation