This answer to this question depends on many different factors. The first factor is the manner of entry in the U.S. Some people enter the U.S. legally with a visa, and then stay beyond the date permitted by the visa (“Overstay”). Some people entered the country without being inspected at all (“Entry Without Inspection”). And some others entered without a visa because they are from a country that participates in the “Visa Waiver Program.” Another factor is how long the client has lived in the U.S. in unlawful status. Also, it depends on who is sponsoring the client for the green card. Yet another factor is whether the client has a history of immigration violations.
As you can see, this is a very complex area of immigration law, and the answer depends on each person’s unique circumstances. The only way to find out for sure if you are eligible for a visa or green card is by consulting with an experienced immigration attorney.