In general, an agent under a financial power of attorney has certain duties. The duty of an agent is to help to manage the affairs of the person for whom the power of attorney was created (principal), A power of attorney is not a license to take the funds of another for your own personal use. If you are the agent, and the principal wants you to transfer assets to others, or put the names of others on bank accounts, then you are only acting to fulfill their wishes, and are not doing anything wrong. However, you have to be careful because many people with POAs are not able to make their own decisions and may not be considered competent enough to ask you to give assets to yourself or others.
In short, an agent is really supposed to manange the assets of another for their benefit. If an agent is taking the funds of the principal for their own benefit, they can run afoul of certain laws, such as those dealing with theft of funds of the elderly. Using a POA as a license to steal the property of the agent for your own use and benefit is a crime. However, if the principal is still competent, that person can direct the POA to give himself assets.
If somebody is abusing a POA, you can bring an action in court to have them removed as the agent. You can also petition the court ot appoint a guardian who will protect the interests of the principal, who can be yourself or any other concerned person. See an estate planning/incapacity planning/elder law attorney and let them know that you believe an agent is abusing her power and not acting in the interests of the principal.
With regard to the speed at which this can occur, that would depend on how quickly you are able to get an appointment with an attorney and how quickly the petition for guardianship can be filed. Also, it would depend on how quickly your petition would be heard in your specific county/jurisdiction, which can vary. Call an attorney asap and you would likely obtain the best result as quickly as possible.
The short answer, is immediately. If you father has competency, he can revoke any POA immediately and remove her authority. It would be prudent if you do this, to notify all major players: banks, insurance companies, dmv that she has been removed.
It's likely that the easy road suggested above is not going to be available and then you are left with the excellent answer already given to get going on a Guardianship. You may also look into some county action for the protection of elderly if he is being taken advantage of financially or otherwise. Best wishes