Since terminal cancer is a fairly common diagnosis and you're likely to get readers with some experience dealing with that, I'd say hew close to reality in this case. Different cancers, and different cases of each type of cancer, will likely play out differently, so there is some wiggle room and leeway, but some things are likely to be universal - lack of energy, perhaps lingering "chemo head," etc. And, as has been said, it's rare that a diagnosis of "a month to live" is precisely on the nose; it means one's in very dire straits and almost certainly has limited time left, but the patient could go earlier or later.
In general, it depends on how central the issue is to the story, how deep you want to research and how accurate you need to get - from "squint at a distance" to "down to the cellular level." (IIRC, the ailment suffered by Tiny Tim in Dickens's "A Chrismas Carol" is chronic literarydeviceitis, unknown in real-world medical circles, but the point of that story wasn't medical accuracy, nor was enough time really spent around him to make accurate details necessary; it was enough the boy was very sickly and that Scrooge's miserly ways were making things worse for him and his family, while generosity would lead to a miracle cure.)