How long to recognize Memorial/Honor Gifts

This really doesn't have anything to do with Tessitura software but I am curious how long other organizations keep Honor and Memorial Gift messages in the program book.  We recognize the donor's name in the donor listing for the full year.  When we started the organization, we listed the memorial/honor messages for the same length of time as the donor's name but then it looked awkward when people made an honor gift in honor of someone's birthday or anniversary and we were still listing it a year later.  We are trying to get a sense of what is the norm.  Do you keep the message in the program for as long as you list the donor's name or do you recognize the messages for a different length of time?