How long does lymphedema generally last?

Life long concern. We try to prevent - with sentinel lymph node biopsies to decrease extent of surgery under the arm but it is always a concern I follow people with the ldex unit to measure their arms with electrical impedance to catch it early before we see it. It can be reversed and limited early in its course. One must avoid any arm infection, BP measurements or venous punctures on the affected arm.
Forever. Unfortunately, once lymphedema develops, it becomes a chronic condition that may come-and-go throughout one's life. Conservative measures which include physical therapy may help minimize the swelling but cannot cure the problem.