My dad has decided to stop chemo treatments. Any idea on how long I realistically have with him??

So as the title says, my dad has cancer and had his last chemo session last Tuesday. He's opting to no longer do chemo due to the impact it has on his body, and the fact he won't be getting better anyway, and therefore doesn't see the point in it. However, whenever I Google about "last chemo sessions," its interpreted as the person in question no longer needing it, and not applying to a person willingly saying they're done with treatment. Thank you in advance and please be kind, I just don't know shit about fuck anymore.