In a nutshell, you should be taking your dog to the vet as often as is necessary.
For routine visits, you should take your dog to the vet once every six months. If biannual visits are too costly for your budget, then you should aim for one yearly check-up.
However, it should be obvious that this is for routine visits to make sure your dog’s overall health and lifestyle are in order. In other words, routine visits are only preventative medicine.
If your dog falls ill, then you shouldn’t be waiting for your annual check-up and should be rushing to the vet immediately. Notice if there are any changes in your dog’s behavior.
If your dog’s appetite has changed drastically, or energy levels seem to have fallen down, or there is any diarrhea, then you should be consulting with a doctor.
You also need to keep an eye out for skin conditions.
If your dog’s fur or skin do not seem like they should, then you should be going to see the vet immediately. Your dog won’t be developing a unique birthmark after half a lifetime, so be prudent and consult with your vet.
Dental disease is also something you need to be mindful of.
If your dog’s breath stinks or there is bleeding in the gums, then you need to reconsider your dog’s diet first.
If that doesn’t help then you need to speak to your vet to rule out any gum or tooth infection.