You can wait as long as you are maintaining employer coverage. Medicare part A will still get assigned to you but (as long as you are not receiving SS benefits) not part B. When you do choose to leave employer coverage make sure you visit SSA.gov and “apply for Medicare part B only”. Keep in mind this may take at least a few weeks and you may have to collect some signatures from your employers HR dept so start this process at least a month before you plan on quitting/ leaving employer coverage.
Each situation is different. I look at the options that you have to make sure you are on the best coverage for you, plus making sure you dont worry about penalties and fees.
As long as you are working and your employer medical insurance plan is considered creditable coverage then you don't necessarily have to apply for Medicare Coverage. In general, what will happen is Medicare Part A will automatically become effective once your turn 65 whether you are still working or not. When getting close to age 65 while also working you should do your due diligence and compare your Medicare options, benefits and costs with your employer plan to ensure you are getting the best plan at the most competitive rate that provides the care you need. Sometimes the employer medical insurance is the best option and other times Medicare is the better option.
If you have worked 10 years or 40 quarters, my suggestion is for you to sign up for Medicare Part A only. It will not have a costs and already having Part A will make it easier for your to get Medicare Part B when the times comes for you to leave your employer coverage in the future.
You can wait with no penalty until you decide to go off the group. You will have Part A hospital but you get the Part B once you decide to go off the group plan and apply for Part B with Social Security or the Dept of SSI You have 8 months to make the transition before you are penalized
If the company you works for has 20 or more employees, you generally do not need to sign up for Medicare Part B. Odds are, your Medicare Part A will start by default the month you turn 65 (unless your birthdate is the first of the month, then it starts the previous month).
If the company you work for has less than 20 employees, and they require you take Medicare Part B, then generally you should drop the employer coverage and go with full Medicare plus either a Medicare Supplement and Part D drug plan or a Medicare Advantage plan.
What you should do is compare the costs and benefits from your employer coverage to what you can get with full Medicare plus a Medicare Supplement and Part D drug plan, or a Medicare Advantage Plan. Many times full Medicare coverage is less expensive than employer coverage and offers better coverage. But sometimes it's not.