No matter the basis upon which you're applying for U.S. lawful permanent residence (a green card), you can expect a long wait. That's largely due to two reasons.
Warning: The coronavirus or COVID-19 pandemic has resulted in especially long delays in every part of the immigration process, owing to staff shortages, backlogs, and at times, U.S. government office closures to in-person visits.
No limits apply to the number of green cards that can be issued to immediate relatives of U.S. citizens. Immediate relatives include the U.S. citizen's parents (if the citizen is over age 21), husband or wife, and unmarried children under the age of 21.
Similarly, no limits currently apply to the number of people who can gain a green card based on asylee or refugee status.
For people who qualify in most other categories, however, annual quotas limit the number of available immigrant visas, otherwise known as green cards. Both family-based and employment preference-based green cards are affected by these quotas.
Green cards allocated annually to employment-based categories, including investors and "special immigrants," number 140,000 worldwide. In the family categories, approximately 480,000 green cards worldwide can be issued each year.
Only 7% of all worldwide preference visas can be given to persons born in any one country. There are, therefore, two separate quotas: one for each country and one that is worldwide. This produces an odd result, because when you multiply the number of countries in the world by seven (the percentage allowed to each country) you get a total that's much larger than 100.
What this means from a practical standpoint is that the 7% allotment to each country is an allowable maximum, not a guaranteed number. Applicants from a single country that has not used up its 7% green card allotment can still be prevented from getting green cards if the worldwide quota has been exhausted.
The bottom line is, the supply of visas often fails to meet the demand, and waiting lists develop in most visa-preference categories. The waits are especially long for people attempting to immigrate from China, Mexico, India, and the Philippines, due to the high demand from those countries.
In addition to the fixed worldwide totals, 50,000 extra green cards are given each year through the ethnic diversity visa or lottery category. Qualifying countries and the number of green cards available to each country are determined each year according to a formula.
The practical effect of these annual visa/green card limits is, in some cases, to completely discourage people from applying. For example, someone who receives a job offer from a U.S. employer, but discovers that in their line of work, the wait for a green card will be ten years, is likely to give up. After all, how many employers will keep a job open for that length of time?
In other cases, the person might decide to sign up for the long wait, if only to see what happens. You can always change your mind. For example, if you have a brother or sister who is a U.S. citizen, that person could start the immigration process on your behalf (by filing USCIS Form I-130), and then when a green card finally becomes available, you could decide whether you are at a stage in your life where you want to pursue it.
Of course, in the family-based preference categories, maintaining your eligibility can be a challenge. If, for example, you are trying to immigrate as the unmarried child of a lawful permanent resident, you will need to make sure not to get married until after you have received your U.S. green card. (Then again, there's been no wait in this category in recent years.)
In some situations, it's possible to shorten your wait for a U.S. green card. For example, if you are the spouse of a lawful permanent resident, and that person becomes a U.S. citizen, then you become an immediate relative and can proceed with your green card application immediately.
Obtaining a U.S. green card is almost always a multi-step process. For example, someone who applied for and received asylum must wait for a year before submitting an "adjustment of status" application, then wait again for USCIS to schedule an interview (which typically takes months), then possibly wait for a decision after the interview.
Someone who applies based on family or an employer must wait while that person or entity files a "petition" on their behalf with USCIS, and it gets approved (which can take many months), and then file paperwork of their own, and then be scheduled for and attend an interview.
At every step, delays are possible, particularly if the government agency gets backlogged, or decides to send the applicant a request for added documents or evidence (an "RFE"). That's why many applicants find it's easiest to hire an attorney to deal with the details.
For more on eligibility and further information on how long you might wait for a green card given the limited numbers, consult with a U.S. immigration lawyer. Or for a summary of the different types of green cards and instructions on handling the immigration application process yourself, see U.S. Immigration Made Easy, by Ilona Bray (Nolo).