Why the answer isn't one universal number
"Voided" doesn't mean the same thing at every insurer — some deny the whole policy after the clause is triggered, others limit which perils (like water damage) still get covered. The only reliable source for your specific house is the policy document or a call to the insurer.
What triggers the clock
Unoccupied generally means no one is living there day to day — a death, a move to care, or a vacant listing all start the same clock, even if furniture is still inside.
The fix costs one phone call
Tell the insurer it's unoccupied and ask about a vacancy endorsement before the clause is ever tested by a claim. It typically costs something, but far less than a denied claim on an empty house.