You are very wise to have stayed away from your 91 year old father as people of his age tend to have a weaker immune system. In addition, symptoms of flu may not be so obvious in this age group.
In general, flu is contagious from 1 day BEFORE symptoms start to approximately 7 days after.
So, as you say, one week is probably long enough to stay away.
However, if your flu was more severe than what you would expect, or if you are still feeling unwell, I would leave it another few days.