Most countries require periodic "maintenance fees" or "annuities" or (most candidly) "taxes" to keep patents in force. In most places, these payments are due each year, but the schedule varies from country to country.
In the U.S., maintenance fees are payable every four years. The fees are said to be "due" at 3 ½, 7 ½ and 11 ½ years after the date of grant, but there is a six month grace period available for a small additional fee. If a fee is not paid, the patent lapses or expires at the 4th, 8th or 12th anniversary of grant. A lapsed patent may be revived if a fee is unintentionally not paid, but the revival fee is large and infringers may get "intervening rights" to practice the invention.