You really will know when Revel has had enough, they do tell you.
If Revel is still enjoying himself and is happy then if he were mine I'd leave him be for a little while, make his last days or weeks as good as you possibly can and be happy for him and with him.
When the time comes and he tells you then don't hesitate, do what needs to be done, with a smile on your face ( for him ) if you can.
It hurts like hell making that decision, it never gets easier no matter how many times you make it, somehow though you do survive it.
Thinking of you, I know how hard it is