I have POA. When and how do i assume financial duties?

If your parent can guide you, ask them what they would like you to do, ie: bill paying, investments, etc.
You need to always keep in mind that these are your parent's assets and try to make the decisions that you think they would make. If you are in doubt, please find a good financial advisor.
I would also recommend opening an online bill pay account. It has saved me so much time and I can keep track of accounts more easily.
good luck