Whatever one can afford. BUT, there is a limit to how much suffering I would cause a pet just to extend his life - especially if he was a very old pet or terminally ill.
I had a Black Lab years ago that had gastric bloat twice. The first time, I spent $1,000.00 on the ER & it was well worth it. Four years later, he got it again, worse. He was 16 at the time & he would have needed surgery & it's unlikely he would survive since he was already near death. The Vet suggested surgery (of course - for $5,000.00) but I declined. It's not right to make a pet suffer just because you don't want to say "Goodbye" to him.
Sometimes a Veterinarian will recommend treatment or diagnostic procedures motivated only by profit. It's easy to use a pet owner's love for their pet to generate income. When my other dog was 15, I took him to the Vet to remove a large cyst on his neck. I wanted to make sure it was harmless, which it was.
The Vet said, "Since he's a senior dog, I recommend doing a complete blood work, urinalysis, etc." I said, "OK, I'll do everything & I don't care about the cost....as long as you guarantee that those tests will turn his clock back & extend his life."
She said, "Ummm....well.....no, we can't do that."