About 20 years ago I heard of a prophecy that when God removed Oral Roberts, Billy Graham and David Wilkerson, that would be the end.
Now, who knows if that is true prophecy or just another rumor started long ago.
It went like this, those 3 got together and made a prayer pact to spread the gospel, not sure if that is true, but it could be.
As an ICU RN, I can tell you that pneumonia at 90 is pretty much the end, so I would expect Billy to die soon.
But remember, God's timing is different then ours.
Don't expect something to happen right away, use biblical examples as a precedent.