The simple answer is: when you are ready. Some women feel it’s right for them personally, while others are not yet comfortable with the idea. The head covering is not because of tzniut (modesty) but because of Yir’at Shamayim (reverence for God) as is the case with a kippah for a male. Wrapping oneself in a tallit, if done with spiritual intention, can be a good personal preparation for entering into prayer. These garbs are expected when carrying out a ritual function, like opening the Ark, holding or wrapping the Torah, leading prayers, or being called to the Torah. If a woman, or a man, receives an aliyah or other honor and does not have a tallit or head covering, there are some to borrow in the hallway, or the gabbai will supply one.