About 10 to 20 days, but this can vary greatly by the time of year you apply, how you apply and your current situation. CMS can even backdate the start date in certain situations, so if you apply on March 30th and ask for an April 1st start date, your application won't be approved for at least 10 days, but your official start date would be April 1st. This is all assuming that you are eligible for Medicare.