Web sites that are updated on a regular basis get more traffic from search engines, and keep web site visitors coming back. 

Many web site owners stop paying attention to their web sites once their sites are up and running "Why should I change anything on my web site?  Nothing about my business has changed in the last 6 months" is a common refrain.

A web site is a work in progress: existing customers come to your web site from time-to-time to see what's new; potential customers will often visit your web site several times before actually buying from you; search engines visit your web site from time-to-time to refresh their indices and record any changes.

How often should content be updated ? 

It depends on the information you are providing.  News items should be updated often enough to remain newsworthy; industry news should be current, and articles should be written regularly. 

A good rule of thumb is to update something on your site at least once a month.

Go ahead and try a regular web site maintenance regime - you will be pleasantly surprised by the results !!