How big can JSON content get in an ASP.NET app?

We have our Web.config file in one app ending like so...

   <system.web.extensions>
      <scripting>
         <webServices>
            <jsonSerialization maxJsonLength="500000">
            </jsonSerialization>
         </webServices>
      </scripting>
   </system.web.extensions>
</configuration>

 
 

...and this is to make sure that JSON which is long enough for our needs may be passed in AJAX. I suppose there is a need to define this in the name of preventing denial of service attacks.