Each month, the State Department issues the Visa Bulletin. The Visa Bulletin tells you the waiting times in each of the employment-based (EB) preference categories. See http://shusterman.com/statedepartment...
However, the dates on the Visa Bulletin do not move forward at the same rate. In fact, sometimes they move backwards. This occurs when there are not enough green cards available to be given out to people in the EB categories. While the supply of green cards is fixed, at least on an annual basis, the demand for green cards is not.
Each year, up to 140,000 people obtain permanent residence in the US in the EB categories. Persons born in a certain country are limited to 7% of the total.
But the preference system is much more complex than it seems.
Sometimes, if it takes too long to immigrate through the EB-3 category, a person will change employers and reapply under the EB-2 category and will be able to retain their original priority date.
Other times, a person in a backlogged category for their country will marry someone from a different country, and use "cross-chargeability" to lessen their waiting time for a green card.
A person waiting for a green card in a backlogged category will sometimes marry a US citizen and adjust status as an immediately relative, thereby shortening the EB line. See http://shusterman.com/greencardsthrou... Or, the person may marry someone from his own country, thereby lengthening the line.
Do not rely exclusively on the Visa Bulletin to tell you how long it will take to get a green card. Instead, learn more about applying for permanent residence through the EB preference categories.