Do you mean for the SKS? Something else?
The status of a folding stock is based on the over-all length of the assembled rifle in its shortest firing configuration. If it is not less than 660mm, it should be legal. Some stocks collapse shorter, but remain legal by blocking the trigger or some other method, so that it can only be fired when over 660mm.
Speaking to the SKS-45, the combined length of the barrel plus the receiver behind the chamber means that most any folding stock should be long enough.
Bullpup stocks are a different matter; please don't ask about those.