The guarantee is 60 days from purchase, and you must contact support within that time.
It is described as a satisfaction guarantee measured from the original purchase date, with full terms on the refund policy and checkout pages. The page does not say whether used bottles qualify.
Our guide on the guarantee terms covers it in more detail.
Read the refund policy before buying. Ask in writing whether opened or finished bottles qualify, who pays return shipping and how refunds are paid. Note the deadline from your order date, not your delivery date.