When To Enroll in a Medicare Supplement Plan. If you are just enrolling in Part A and Part B you have a six-month time frame that is called the open enrollment period. During this time you may enroll in a Medigap plan and be guaranteed coverage regardless of any pre-existing conditions you might have. Therefore it is a good idea to start your Medicare supplement coverage the same day as your effective dates of Part A and B Medicare. Anyone who chooses to opt-out of Part B Medicare due to having employer health coverage will still get this open enrollment period when they do eventually enroll in Part B Medicare. Once you decide on a plan than the next step is to get instant quotes online to compare the premiums from top companies in your area. We can help you every step of the way to make sure you understand the coverage, as well as help you apply with the best company. We help make Medicare easy!