My senior dog has a spinal tumor. How much time does he have?

I am so very sorry to hear your baby has a tumor! That is devastating! Depending on what type of tumor it is and it's location (if it is in the bone vs in the spinal cord) can vary the average life expectancy of the animal, so it is really difficult to say without knowing these things. There are definitely treatments/medications that can help improve Caesar's quality of life for as long as possible. Additionally, some tumors respond favorably to radiation treatment or potentially chemotherapy as well, so that may or may not be an option for Caesar. If you aren't doing further treatments, then often adding in steroids to reduce swelling and inflammation can potentially make Caesar's quality of life as good as possible. I would discuss medication and expectations with your vet, as they will hopefully be able to give you more information based on what they are seeing in the x-ray. I would also focus on how he is doing day to day and how comfortable he is. Not every dog "reads the book" on how cancer progression is suppose to go and we always hope for longer time than the average. I would consider getting area rugs or carpet runners to give Caesar more grip when he is walking. Hard wood floors and tile can be difficult for any older dog to walk around on, so giving him more traction may help. I would also discuss pain management with your vet. Sometimes if they are more painful, we can adjust their medication for the "bad" days. Ultimately we look for good quality of life. If Caesar quality of life is starting to decline ("bad" days outnumber the "good" days), then I would consider humane euthanasia. It's never an easy decision for any owner (vets included!) but ultimately it is the last kind thing you can do when there is nothing else you can do for him. Again, I am so very sorry for you and your boy!