Changing workout, sets and reps

punkeson

Hi
I've been doing a Bret Contreras workout for the last 8 weeks. The plan is broken down into categories such as vertical push, horizontal pull etc, and you choose one exercise from each category each time you workout. I mainly do 3 sets of 8 reps for each exercise.
I know that you should always keep your body guessing and change your workout from time to time - I just wondered, do people change their sets and reps each time they workout, or is it more a case of changing the whole workout every couple of months or so? I love this workout but have got into a naughty habit of doing my favourite exercises each time! I don't really mind as I enjoy the workout but I'm aware I'm probably no longer getting optimal results.
Thanks
Tina