Can the insurer or plan cancel or rescind at any time?
Generally state law and the policy itself provide that the insurer has only 2 years from the date of application to rescind. If discovery of the omission or misrepresentation occurs after the passage of the 2 year period of "contestability" the insurer is generally out of luck and can not cancel the policy. After that, an insurer may only be able to contest a claim on the basis of actual, intentional fraud on the part of the policyholder, and even that may be severely limited.
Because some insurers engaged in "post-claim underwriting", a practice in which the insurer would write a policy and take the premium, and only after a claim was submitted look to "find a way out of it", many states have statutes which prohibit "post claims underwriting." In those states an insurer or plan cannot rescind or cancel a policy once an insured has filed a claim for benefits unless the insurer or plan can prove intentional fraud.