That depends on a number of factors, including how soon they need to fill the position, how many other canidates they are interviewing, whether they've secured the budget to hire a new person, and so on.
If you haven't already done so, send thank-you notes to each person with whom you interviewed. Do this immediately - it will help you stand out more from the other candidates they've interviewed who ignore this bit of etiquette.
If you don't have their contact info, the hiring manager's assistant should be able to help you (and include a thank-you note for him or her, too).
Make sure you spell their names correctly, thank them for their time, and reiterate your interest in the position and the company. You may also use this opportunity to address some issue that may not have been covered adequately during the interview - just keep it succinct.
You can hand-deliver the notes for an extra touch, which will also make sure they get there faster than via mail.
Give them time to have received the notes - if you haven't heard from them in a week or so, call the hiring manager and follow up, asking if there is any additional information they need to make their decision easier.
To your success,
David B. Wright
Author, Get A Job! Your Guide to Making Successful Career Moves
http://www.thegetajobbook.com/
http://jobs.therecruiterslounge.com/