at a year of age my lab is having 60 mins a day divided into 30 mins in the morning and 30 mins walk in the afternoon.  When can I increase her walks to an hour in the morning and an hour in the afternoon?  At what stage do you stop following the  5 minute rule?

You can start gradually increasing it from about now - obviously not imediately doubling the amount, but gentle increments, and watch how she copes with it.
:)