Immigration visa allows the recipient to obtain a Legal Permanent Resident Status, also known as a Green Card, and could be obtained in one of three ways:
The US Congress sets quotas for the total number of visas in each category and for the number of visas for each country. Visas are distributed according to those quotas as well as according to preference category (depends on degree of relation, age and marital status for family based petition and professional qualities for employment based petition). As a result, the rate of climbing up the visa queue is a function of a complicated equation with multiple factors. Time approximation could be somewhat predicted by checking the Bulletin published by USCIS for different countries at http://travel.state.gov/content/visas/en/law-and-policy/bulletin.html
A spouse or a child accompanying the principal immigrant obtain same level of preference and same spot in the visa queue.
The clock for the position in line begins to tick at the moment of the first document submission.