You can apply for a Medicare Supplement plan any time. However, the best time to buy a Medigap plan is during your 6-month Medicare Supplement Open Enrollment Period.
Your Medicare Supplement Open Enrollment Period starts the first month you have Part B and you're age 65 or older. The big advantage is you can buy any Medicare Supplement plan sold in your state without medical underwriting. So even if you have health problems, you can buy any Medicare Supplement plan for the same price as people with good health during this time period.
Some states have additional Open Enrollment Periods including those for people under age 65.