2 to 3 months. Originally, the Centers for Medicare and Medicaid Services (CMS) noted that it generally takes about
two months from the time your Medicare drug plan submits the request for the premium deduction to start. Since several months are required before the deductions begin, the first-time monthly Medicare plan premiums are withheld from your Social Security benefit, two (or three) monthly premium payments will be withheld at the same time. Once the automatic premium deductions begin, Social Security will deduct only the cost of one monthly premium payment from your monthly Social Security benefit.
In December 2017, the Centers for Medicare and
Medicaid Services (CMS) issued revised guidance on the issue of having
your monthly premiums automatically deducted from your Social Security
benefit check, noting,
"[Social Security] could take up to 3 months from
the time you request premium withhold before you start seeing premiums
withheld from your Social Security payment." [emphasis added]
CMS also added some
clarification about paying past or unpaid months, noting that,
"[d]epending on when the plan gets your enrollment request, premiums
might be withheld back to the start of your enrollment in the plan [as
noted above]. In cases where premiums weren’t withheld from your Social
Security payment until 1 or 2 months after you enrolled in a Medicare
drug plan, you’ll get a bill for the months your drug plans premiums
weren’t withheld. You’ll need to pay your drug plan’s monthly premium
directly to your plan. Your Medicare drug plan will let you know If there’s difficulty withholding premiums from your Social Security payment." [link added]
So
it is possible that the Social Security administration will not
withhold back premiums and instead, your Medicare Part D plan will send
you a bill for those months that were not paid through your Social
Security benefit check. In such a situation, you need to pay these unpaid
premium directly to your Medicare plan.
See also:
Why am I not allowed to have my Medicare Part D premiums automatically deducted from my monthly Social Security check?
(sources: U.S. Department of Health & Human Services and "Withholding Medicare Prescription Drug Premium from Your Social
Security Payment" (Revised December 2017 & May 2019), CMS Product No. 11400)