If you take a critical illness cover policy out, the typical survival date after being diagnosed with an illness is 14 days. If you were not to survive the full 14 days form date of diagnosis you would not be paid out the insurance benefits.
An insurance company will not cover pre existing conditions, so on your applications you will need to disclose if you have any kind of illness no matter how material you think it is this is the basis of your Critical Illness insurance policy being activated. If you have not fully disclosed again the benefits of your policy will not be paid out.
Throughout your policy period, if you make any changes to your life style such as occupations, address, family history etc. you must inform the insurance company. This is also the basis of your contract with the insurance company. Again if this is not adhered to your policy benefits will not be paid.