It actually doesn't even have to be at 0 hunger. It simply has to be at "dying" which I'm pretty sure starts at 20.
After the point when the pet reaches dying status, it takes ten days to die. Fifteen only applies to losing the name once the pet is inside the graveyard.
So your pet should die any day now if it was at 0 hunger by the 26th or so, meaning it was at 20 around the 24th. Probably two or three days left.