How long should we warm compress my moms dogs spine before we give up?

live with my mom. She has a 12 year old pitbull with really bad arthritis im his spine. The vet instructed us to take a heating pad, put it on his back, and massage it for 15 mins twice a day. I did it this morning for him. I just got home, and my mom and her dog are asleep, and I'd rather not wake them up. On the other hand, I don't want to neglect him, and possibly cause him pain..

How long should we warm compress my moms dogs spine before we give up?
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