Dad has heart, lung, liver and renal failure (renal function under 10%). Drs have now found primary liver cancer. How long left? Days/weeks/months?

Soon. I'm sorry you are going through this. It doesn't sound like there is much time left, probably 'weeks" timeframe depending on how aggressive supportive care is regarding IV fluids, dialysis, medications, and respiratory support with oxygen or a breathing machine. There is of course no way to predict for sure, and his doctor will be ae to give a better answer. Hang in there.