The term 'one year' appears 14 times in the June 2015 US DoD Law of War Manual. Six refer to wounded PoWs. One to land mines. Six refer to the application of the Geneva Convention after the cessation of hostilities in the case of occupied territories. One refers to a prison sentence.
From what I can tell the Geneva Convention (GC) statements have been misunderstood:
"11.3.2 Duration of GC Obligations in the Case of Occupied Territory. In the home territory of parties to the conflict, the application of the GC shall cease on the general close of military operations.86 In the case of occupied territory, the application of the GC shall cease one year after the general close of military operations; however, the Occupying Power shall be bound, for the duration of the occupation, to the extent that such State exercises the functions of government in such territory, by the provisions of the following Articles of the GC: …" p.751
I've looked at the original document and I can find no reference to 'the illegitimate government must be in power for a year before the military can legally remove them'. This statement is absurd on its face because military strategy to remove an occupying power is based on an assessment on the battlefield, not on a calendar. We can't land at Inchon to drive out the NorKs because the manual says we have to wait until the one year anniversary of the invasion? It's BS. Sorry.