Green cards can be issued to any person who has been living in the US for at least 5 years. Though not actual citizenship, it does give the person the status of a lawful resident of the United States, and the benefits and privileges that come along.
People who have been living in the US on the grounds of political asylum can apply for a green card after completing 4 years of stay in the country, while people who are married to US citizens can apply for it after completing 3 years of residency. The processing time depends on the kind of sponsorship that the immigrant is provided.
You should keep in mind that along with correct petitions of sponsorship, the results of biometric submissions and your personal interview hold a lot of importance in the process.
It also depends on the birth country of the applicant, the method chosen for application, case load that the immigration office has, and when the process of green card application began.
There are various types of immigration sponsorships, such as family-sponsored immigration, employment-based immigration, special immigration, political asylum, and other types of exceptional immigration considerations.
Family-sponsored Green Card
You need to understand that there are two steps involved in this case: (i) the preference petition filing with Form I-130 and (ii) the immigration status of the application. The filing date for Form I-130 is the Priority Date of the immigrant applicant.
Only when the priority date is marked as current, can you think of filing for immigrant status. There are two ways through which you can file for immigration status: (i) overseas counselor processing and (ii) adjustment of immigration status.
It is important to understand that family based immigration for the immediate relatives, like spouse, minor children, and parents of a US citizen, who is over 21 years of age, can get a green card in 6 - 9 months.
Approval of all the other types of family-sponsored immigration cases depend on the clearance of document backlogs and the country of charge. If you look at the current status, there is waiting period of about 2 - 14 years.
Employment-based Green Card
Like the immediate relatives of a US citizen or green card holder, employees who apply for green card under employment first preference may get a green card within 6 months.
Processing of this type of green card involves three steps: (i) Foreign Labor Certification Application (PERM), (ii) Immigrant Preference Petition (Form I-140), and (iii) Adjustment of Status. The PERM approval takes about 20 days - 14 months and Form I-140 may take around 15 days - 6 months.
The application date for the immigration benefits is counted as the priority date of the immigrant applicant. From then on, adjustment of status may take about 6 months.
This process has even taken 5 years in some extreme cases! Apart from the employment first preference, all the other types of employment-based immigration take a lot of time to get approved. Current immigration visa backlog in this category is between 5 to 9 years!
Other Immigration Types
Apart from the family-sponsored and employment-based immigration, all the other eligibility types have least amount of immigration visa backlog. There is no waiting period for people applying for green card under political asylum category.
Categories like diversity immigrant (DV) and refugees have an annual green card issuing quota of 50,000 and 70,000 respectively. However, there is no backlog there, and hence, you are up for immediate success in these categories.
It's also worth noting that the US government grants about 300,000 - 500,000 green cards to immediate relatives, while around 40,000 employment first preference applicants get green card on an annual basis. All the other categories has varied amount of approvals, depending on the annual quotas and immigrant visa backlogs.
As you can see, the processing time for a green card cannot be specified. So the best thing to do is to follow the process, be clear on your part, and wait until your application is approved by the authorities! Good luck!