When you purchase the insurance generally has some impact on what is covered or how much some things are covered. We were able to get coverage for medical evacuation (the only thing I really am worried about when I cruise) just a couple of weeks prior to the cruise, no problem. Coverage for things like financial default of your air carrier, by comparison, is generally not offered unless you buy your insurance immediately after first booking your trip. (Different companies and policies will differ. See each policies T&Cs for details.)