My aunt has just died, and did not have children. I have started to say kaddish, (I can as my father is no longer alive. How long should I say kaddish, a month or a year? And how many times a day? She was the last sibling of my father, and as she could herself not say kaddish on the yahrzeits of her parents, and siblings, I did so. Now that she has died, should I stop, as I am not doing it on her behalf anymore.

Shalom U'verachah, There is no obligation to say kaddish for someone who had no children or family, but it is a good custom and if you decide to do so you should say kaddish for the year as if she had a child. If you said kaddish for other family members you should continue to do so as their souls get pleasure in the next world when you say kaddish here. Kol Tuv