Depends on the weather and where you live. I know some people this year didn't stop because the weather was so mild. I just got a message from my vet that we should start early this year (actually just gave my pup his flea/tick stuff this morning) Last year he started in May and his last does was November I think. My vet mentioned when the weather is consistently around 4 degrees (40 F... I think) that's a good time to start.
All that said I would just give your vet a call and see what they say.