There is no one answer to this question, as each dog's experience with hallucinations will vary. However, some possible reasons why a dog might have frequent hallucinations include:
Fly-snapping syndrome. This condition is the result of canine epilepsy, and can cause intense and recurrent hallucinations.
Vitreous floaters. These are small, round objects that sometimes float in front of your dog's eyes. They're most likely harmless, but can trigger visual hallucinations in some dogs.
Seizure disorders. A variety of different seizure disorders can cause hallucinations, including petit mal and grand mal seizures.
Degenerative diseases. Many degenerative diseases can cause changes in the way your dog sees the world around him, which could lead to recurrent hallucinations.