how long have to wait before getting the green card?

If you are a US citizen or a lawful permanent resident, you may then petition for your immediate relatives for a green card. Obtaining a Green Card for your family is a two step process. The first step is filing the Family Immigration Petition. This is filed to establish a qualifying relationship between yourself and your immediate relative. The second step involves applying for a green card. If the family member is inside the United States, then the applicant may qualify to adjust status to permanent resident without returning to their home country. Or if the family member is outside the United States, then the applicant must file the petition at the United States consulate or through an US Embassy that has jurisdiction over their foreign place of residence.

normally, if you enter the US on an immigrant visa it takes several months to get the actual card. It's possible to go back after you enter, but if you stay out of the US for over 12 months, you'll lose your status.

If you are sponsored by a US Citizen or Permanent residents family members means, you can ask them to file a petition for a green card for you.Here i have mentioned the average time you should wait before you get the green card with your preference relative:

1.Unmarried children of U.S.Citizen- Seven Years
2.Spouses or Children of Permanent Residents - Four Years for Spouses and for minor children(2A), Eight Years for Adult Children Age 21 or over.
3.Married Children of U.S.Citizens- Ten Years
4.Brothers and Sisters of U.S.Citizens - Eleven Years