Most countries require periodic "maintenance fees", "annuities" or "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 due 3 ½, 7 ½ and 11 ½ years after the date of grant; a six month grace period available.
If a fee is not paid, the patent lapses or expires at the 4th, 8th or 12th anniversary of grant.