Hello, this is Dr Benedetta, and I would like to help.
I am truly sorry this is happening to your dog, it is a tough situation.
To have an accurate estimate of the situation, the best thing would be to consult with a neurologist and have an MRI done.
This will give you better answers. There are also different antiseizure medications that can work better than phenobarb, and give less side effects. Again, a neurologist would be the best person to help.
Steroids are useful to decrease inflammation and sometimes reduce the size of a tumor for some time, but are not curative, at best they give respite. The positive effect is usually pretty quick to show, then it plateaus, when it happens at all.
Having said all that, unfortunately your dog has an incurable disease, and is not responding well to palliative medication.
There is no answer to your question" when is it time?" it varies for each dog and each owner. If he looks miserable, has no quality of life and is not likely to improve, it might be kinder to make peace with this horrible situation and let him go.
I am so very sorry for all you are going though, it is tough, and it is a difficult decision!
If I can be of further help, please let let me know!
Benedetta Sarno, DVM