Looking at an AVA calender we have on our wall at work (no not a vets) Pro Heart SR12 is now recommending 1 treatment 12 weeks, 1 treatment at 6 months, again at 15 months and annually thereafter That's if your dog does't die first from all the chemicals that have been injected into it. It also says 'use a dose rate based on the dogs body weight at the time of treatment.
Personally I would never use Proheart Injections again from all the bad write-ups on it. Plus it contains Ivermectin which shelties can be allergic to. We use Interceptor. No Ivermectin.
Jonene