If you don't sign up for Part D when first eligible and go 63 consecutive days without creditable drug coverage, you might pay a late enrollment penalty added to your monthly premium. This amount can only be determined by CMS (Medicare), but is calculated by multiplying the average monthly premium by 1%. You can estimate by basing your math on a $35 per month average premium and then multiply by the number of months you went without Part D coverage. This penalty is assessed monthly for as long as you carry any type of Medicare Part D coverage and will be taken directly out of your Social Security check.