The Ashkenazi custom is to stop saying Kaddish for a close family member 11 months after their death. Although the period of mourning is a year, there is a tradition that judgment for the righteous takes 11 months, whereas for the guilty it lasts a year (Talmud Rosh Hashanah 17a). We want to give our relatives the benefit of any doubt and so stop after 11 months. (The Sephardi custom, based on the teachings of the Ari, is to say Kaddish for 12 months minus one week).
Many report a feeling of emptiness on stopping saying Kaddish. A continuing cord of connection with the departed person ceases. Whereas the beginning of saying Kaddish is accompanied by communal recognition and support, the end tends to be lonely and unacknowledged. In some American shuls, including Rabbi Avi Weiss’s in Riverdale, I have seen the rabbi speak briefly about the mourner and the person mourned at the end of a weekday service when someone stopped saying Kaddish. I have never seen this in Britain but it seems like a compassionate and considerate way of marking an important moment for the mourner.