Within 2 to 3 months.  Depending on any administrative issues, you can usually expect your Social Security check premium deductions to begin within 2 to 3 months after the start of a new Medicare plan year or the enrollment into a new Medicare Part D or Medicare Advantage plan.

In May 2019, the Centers for Medicare and Medicaid Services (CMS) issued guidance on the topic 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.  Depending on when the plan gets your enrollment request, premiums might be withheld back to the start of your enrollment in the plan. 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 plan’s 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]

Paying for past, unpaid premiums

Please note: Your first Social Security deduction may include any previously unpaid premiums (see above).  For instance, if your Medicare Part D prescription drug plan premium deductions begin in February, the first deduction will include both January and February monthly premium payments – so be sure to budget for less monthly income from your Social Security check in February.

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 would pay these unpaid premiums directly to your Medicare plan.

See also: "Why am I no longer allowed to have my Medicare Part D premiums automatically deducted from my monthly Social Security check?"

Time may be needed to correcting premium payments

Please also note:  If you changed your Medicare plan late during last-year's Annual Enrollment Period (such as on December 6th) or you used the Medicare Advantage Open Enrollment Period (MA-OEP) to change your Medicare Advantage plan or dropped your Medicare Advantage plan and added stand-alone Medicare Part D prescription drug coverage, the Social Security Administration may need a little more administrative time to begin deducting the correct amount from your Social Security check.

For example, if you dropped your Medicare Advantage plan in January and then join a stand-alone Medicare Part D plan also in January, you may see payments for your old Medicare Advantage plan deducted in January and February - then in March, the February Medicare Advantage plan premium payment paid back (or reversed) and the February and March premium payments for your new Medicare Part D plan taken out of your Social Security check.

As noted by CMS back in 2009,

"[i]f you have your Medicare drug plan’s premium withheld from your Social Security payment (called “premium withhold”) and change drug plans, it may take up to 3 months before the correct amount is withheld. During this time, the 2010 premium amount for your old plan [and not your newly chosen Medicare plan] may continue to be withheld from your Social Security payment."

Example 1 - Switching to a lower-premium Medicare plan

"Mr. Jones switches to a [new Medicare] plan for 2010 with a $20 monthly premium. His old plan’s premium is $35 in 2010. It may take 3 months before his new premium is withheld. During this time, $105 [3 * $35] was withheld from Mr. Jones’ Social Security payment, but only $60 [3 * $20] should have been withheld. Once Medicare’s records are updated, Mr. Jones will get a refund of $45 ($105-$60) from Social Security." [emphasis added]

Example 2 - Switching to a higher-premium Medicare plan

"Mrs. Smith switches to a plan for 2010 with a $30 monthly premium. Her old plan’s premium is $20 in 2010.  It takes 3 months before her new premium is withheld.  During this time, $60 [3 * $20] was deducted from Mrs. Smith’s Social Security payment, but $90 [3 * $30] should have been withheld. Once Medicare’s records are updated, Mrs. Smith will have a deduction from her Social Security payment for the additional $30 ($90-$60) owed." [emphasis added]

And the worst case scenario of paying back-premiums is . . .

Back in the early years of the Medicare Part D program (2007), we heard from people who waited up to 10 months before their Medicare Part D plan premiums were deducted from their Social Security benefit check and then, all at one time, the complete balance of Medicare plan premiums was paid from a single SSA check.

Related Question:  My Medicare plan premiums changed from last year, do I need to notify the Social Security Administration about the premium change?

No.  You do not need to notify the Social Security Administration about your Medicare Part D or Medicare Advantage plan premium change. If you have elected to have your monthly premiums automatically deducted from your Social Security check, any increase (or decrease) in monthly plan premiums will automatically be adjusted by Social Security.

Please also see the other related questions:

• What if my monthly Social Security benefit isn't enough to cover multiple premiums at one time?

• Why am I no longer allowed to have my Medicare Part D premiums automatically deducted from my monthly Social Security check?


Additional sources,
"Withholding Medicare Prescription Drug Premium from Your 2018 Social Security Payment" (revised December 2017),
https://www.medicare.gov/Pubs/pdf/11400-Withholding-Medicare-Drug-Premium.pdf (revised May 2019)