This cover appears to be more of a memorial cover than a mourning cover. I am puzzled by the fact that it is issued on the 39th anniversary of the death of the person being memorialized. The individual was an interesting historical figure.
There is no set amount of time during which a person would have used mourning stationery - it was for however long the person considered themselves in mourning. In an extreme example Queen Victoria went into mourning on the death of her husband, Prince Albert in 1861 and never stopped mourning for the rest of her 40 more years of life. She wore black all throughout that time. A mourning cover might have included a death notice or an invitation to a funeral or wake, but just as likely it could have been used by that person as their regular stationery for at least a year after such a death (considered proper for widows in most places during those times) or they could have been like Queen Victoria and used it as their regular stationery for 40 more years after such a death. It was up to the individual as to how long they wanted to let the world know that they were mourning a loved one.