One of my good mates was diagnosed with stage 4 stomach cancer in February last year. Docs told him he probably had 6 months to live and he made it to Christmas, which was remarkable given how ill he was. He deteriorated very quickly after diagnosis, then had chemo that he responded brilliantly too and was almost his normal self for a few months. Eventually though the chemo stopped working and sadly he declined very quickly (over the space of a few days).
Obviously everyone is different but the 5 year survival rate is less than 5% for stage 4 stomach cancer