Philipina Married to a U.S. Citizen

Hi,
I know a Philipina woman who married a U.S. Citizen. They have been married for atleast 3 years. What happens once married for her to become a citizen? What is the time frame, etc, for her to have legal permanent status. I only ask because I need to verify some information she has given my father-in-law. Thank you.

If she has been married to a USC for three year, and has not yet obtained Legal Permanent Residency, then she may have entered without a visa, or have a prior order of Removal/deportation, or any number of reasons that render her inadmissible (unable to obtain Legal Permanent Residency.)