The USCIS usually processes cases in the order they are received. For each type of application or petition there are specific workload processing goals. The USCIS WebSite has a table for determining how long they are taking to process an application or petition at a particular office. If you have already filed an application or petition, you can get an idea of how much longer it will take to process your case.
Use the menu at the bottom of the page to find the local office or service center handling your case, then click the relating Processing Dates button. This brings up a chart showing the form number, form name, and processing times for all of the forms processed at that office.
USCIS processing time information for the following can be found by clicking here.