Hello,
I don't know anything about stomach or liver cancer, sorry, but I imagine the time wouldn't be long.
My husband died of a brain tumour which took him in 5 1/2 months. Ver quickly after the DX he went into denial and never really came out of it. It was very difficult as we couldn't discuss things re. the funeral or anything related to death. At one point he got really angry with me and accused me of being the only one who thought he was going to die.
I would advise you to make sure your father has done his will and has power of attorneys in order, this is so important.
I hope you have support, it makes all the difference.
Bye Anita