When your husband retires, you are losing your health coverage, so that is a qualifying event. You should enroll in COBRA for 18 months, as long as he does not work for a church or small employer.
You are probably only entitled to 18 months of COBRA, because of IRS Revenue Ruling 2004-22.
A spouse is not entitled to an extension of the COBRA maximum coverage period to 36 months because the employee's entitlement to Medicare could not be a qualifying event. Eligibility for medicare is not a triggering event for COBRA.http://www.cobrahealth.com/medicare.htm