It varies for everybody. It can take as long as a year (or in rare cases more) for some people in some cases.
Did you check the USCIS processing times for your local office? (Select your local office under "Field Office" and look at the processing time for I-485. Remember that the times are not constantly updated so the date you see is the filing dates they were processing at the time the processing times were last reported.) Is your application beyond the timeframe for the office? Even if it is beyond the timeframe for that office, your particular case could be slower than the average because they are doing extra security checks or something.
If your case becomes really behind, you can contact your congressman or senator and/or the CIS Ombudsman to see if they can help move it along. But if it's within normal processing times, they're not likely going to do anything.
Hopefully, you applied for an EAD (with I-765) and Advance Parole (with I-131) with your Adjustment of Status application. They are free to apply for and with them you can work and travel and has most of the benefits of permanent residency, so it doesn't matter as much how long it takes for your AOS to be approved. If you haven't applied for them yet, you can still apply for them for free; but it takes about 90 days from applying to get them.