I enrolled in part A at age 65. Now I want to enroll in Part B. Is there a specific time to enroll?

Many employers do not pay retirement supplemental insurance benefits anymore, today more of them do not.
All that changed in the 1990's when corporations were getting gobbled up by bigger companies and those employees from the smaller companies lost out unless they negotiated items like this pre buy out. Else it just stopped. 
The person is indeed lucky if your last employer stills pays something to take care of you after SS kicks in. 

To Hanson. If your Mom has limited income and savings please check out your State's Medicare Savings Program. It is not Medicaid, but could help pay for your Mom's Medicare premium, deductibles, and other out of pocket costs.

I brought my mother back to the USA two years ago when she turned 90. Social security told me I must pay a penalty for every year she did not sign up since the age of 65 even though she was living in Switzerland and had health insurance there and never knew she would return to this country. She couldn't get out of the penalty so she pays around $380 for Medicare all parts ! It's a ridiculous rule !!!

I agree though. Calling Medicare is probably a good idea. Especially with your background living overseas.

I think someone mentioned on this site that if your prove you were covered by an employer it makes a difference. You don't have to sign up for a supplementary thru Medicare. In my husbands instance, his health insurance carried thru into retirement. Once he was 65 it became a supplimental. Same with my Mom, Dads employer continued the insurance into retirement. There is a deduction, I think, if you choose a suppliment thru them. Need to shop around.

Are you back in the US?

If you didn't enroll within 90 days of turning 65, then you must wait for the enrollment period...Nov.-Dec for a policy that becomes effective in January.

If you did t enroll in Medicare part D when you first become eligible, then you will have a penality that you must pay every year. The longer you wait..the larger the penality. Again..enrollment is Nov-Dec for a start date January 1

As for the cost.   There is a wide array of plans.  You need to figure out what deductible you are willing to go with..what co-pay, etc.   check with a few insurance companies.   The cost will also vary by your current age, and the insurance company.

For example..I have a plan F.  No co-pay, no deductibles.   With Bankerlife the plan would cost $230 per month.  With United health the plan would cost $184 per month.    Same coverage.   So, shop around.

There is a heck of a lot of information on Medicare's own website, have you looked there? Best to get it from the horse's mouth, I'd have thought.