I think what you are referring to is the deadline to buy insurance which covers pre-existing conditions. It can range from 7 to 21 days from the time you made the booking. Depends on the insurer. 14 days is common. After that point you can still buy insurance but it may not cover you if you have to cancel the cruise, etc., because (for example) you had a heart attack and it was known that you had heart disease when you booked the cruise, or within X number of days prior to booking.
I don't buy insurance from the cruise lines. Jim is right - check out your options at insuremytrip. Agents also sell policies from the private insurance companies, which is another reason to consider using an agent for future cruises.