I just play it by ear. There's no right/wrong time to use it and it also totally depends on the dog.
Throughout the year, I usually start with the slicker brush. If I notice that there's a lot of undercoat coming out, then I'll switch to the Furminator. I find that if the Furminator is needed, its better to start with it, then finish with another brush.
I also use the Furminator to thin their pants, when they get big and fluffy. Again, I use it as needed. I rarely have to think with scissors (Have only done it once with Ginny fluffy butt).
In the early summer they blow coat and the Furminator is definitely needed. At this time of year its obvious its needed because there are sheltie tumbleweeds floating around the house, fur comes out when you pet them and their coats get tangly as the loose undercoat comes out.
During the summer I'll pull out the furminator every couple of weeks to ensure there's no extra undercoat to make them hot. Some times its needed, some times its not.