It starts the moment that an attorney/client relationship is established, and the client communicates any confidential information to the lawyer. This is usually the moment that the lawyer agrees to represent the client.
It may also apply to disclosures made by the client in the course of the lawyer assessing whether s/he will represent that client.
It exists even if the lawyer is never paid, and may not be held hostage against payment. It applies to all information communicated between attorney and client while the relationship is in place, with a few limited exceptions.
Advice on how to commit a crime is not covered. Billing information may not be covered. Information communicated in the presence of a third party is usually not covered. Information that the lawyer is directed to convey to a third party is not covered. Information that was already public is, I believe, not covered.
The client may waive the privilege as to any particular item of information, should the client so choose.