Don't wait - tell them now.
How often do we watch films and see someone saying something meaningful with their last breath, something they have been keeping to themselves for so long? Why did they wait?! If it's worth saying, say it now.
In our house whenever someone is leaving the house, we meet at the door, hug, kiss, and tell them that we love them, because who knows if they're coming home? You may think that's a morbid thought, but there's no misunderstanding in our house - we all know we are loved! And it means that, if something awful happens, we know that our last interaction with them was good.
So say it now. Don't wait.