Is it to insure your life or your wife's? Whoever it is for must of course declare their true medical situation to ensure that a payout happens.
You can insure a life for a specified length of time (e.g. we insured ours until our youngest child was 21) or up until death.
It might be wise to go to an insurance broker who has access to the whole market in order to get the greatest choice of the many firms that are out there.