Do you have the levels set up in your built? If so the level you are on is stored under Application.loadedLevel being the last level loaded. In that case just write an if(Application.loadedLevel == 6) for starting your countdown.
If you don't have the levels set up in your built do the following:
Go to File > Go to Built Settings
Now drag and drop the 'scenes' you have in your project which are the levels in the big box. Arrange them so that the startoutlevel (often the main menu) is level zero, etc. So level six will then be number six and this all should work. :)
http://unity3d.com/support/documentation/ScriptReference/Application-loadedLevel.html