I don't think we were supposed to know but it would explain why Sami, Carrie, along with Bo/Hope are returning next year. They will probably write out John before this(my guess is that it will be hard for him to continue working obviously). So he will probably be killed off before then.
I kind of hope they were able to film some stuff with him. If they knew it was terminal then they might've felt it too morbid to film John's death. Perhaps just him leaving town for some reason. He'll have an off-screen death like they did with Victor.
Given Days bizarre tape to air schedule my guess is we will see John for the next few months before this happens.
Yeah, it could be six months before it even starts to play out on screen. I kinda hate DOOL's film schedule. They could waste a ton of time on something fans utterly hate vs. giving up on something before they get proper feedback from viewers on it.