How long should a small dog take antihistamines for?

I am living in Spain but the vets are shit over here and can never give a good answer.

Only when they need it. I have a small dog that takes a tsp of children's benadryl only when he has symptoms (red eyes and sneezing). I was never given a time frame (how long). You could call your vet and ask via phone how long- even though you don't like the vet, I'm sure they would give you that piece of information.