Here's a common scenario: Your Silicon Valley employer sponsors you for an H-1B visa, and then sponsors you for an employment-based green card. How long it takes to actually get your green card depends on your priority date (the date your employer submitted the first application), your visa preference category, and your country of chargeability (usually your country of birth). It can take a relatively short time, to several years.
Each year 140,000 employment-based green cards can be issued. Over the years well over 140,000 annually have applied, resulting in a long queue. Once your priority date becomes "current", you can then apply for a green card. About 85% of the 140,000 green cards go to people currently living in the U.S. who apply for an "adjustment of status". The remaining 15% go to people seeking to immigrate from abroad. The USCIS estimates that there are currently 234,000 pending employment-based adjustment of status applications pending.
Until recently, the only way to estimate how long it would take for your priority date to become "current" was to view the Department of State's monthly Visa Bulletin. The Bulletin shows the current priority dates within each preference category. You can view the Visa Bulletins over several months and track the movement in particular categories.
Recently the USCIS started providing more detailed information by publishing sets of charts. The charts track the USCIS' total pending inventory of applications for employment-based adjustment of status. The charts show how many pending adjustment of status applications in each preference category have priority dates in a given month and year. You can use the chart to determine how many applicants in your preference category are ahead of you in line for a visa number by adding up the number of cases with an earlier priority date than your own.
For example, the chart below shows the 1st, 2nd, and 3rd preference categories. Suppose your priority date is January 2003, and you are approved for 2nd preference. Using the chart below you can see that there are six other adjustment of status applications with the same priority date. You can add up how many 2nd preference applicants have an earlier priority date by adding up all the numbers starting at the beginning of the table, and ending with December 2002. In this case, there would be 130 applications with a priority date earlier than yours.
http://www.scribd.com/doc/21070389/I-485-USCIS-Partial-Chart
I-485 USCIS Partial Chart