So far as I know, the Church does not give a specific timetable. As a rule of thumb though, cremains should be kept only for a serious and objective reason, and not because of personal attachment to or desire for the cremains. So, for example, temporarily keeping the cremains at home until you are able to raise the necessary funds to have them properly interred may well be legitimate. Keeping the cremains because you cannot bear to part with your relative’s ashes is not legitimate because it turns the bodily remains of a human person into the personal property of another.
