He remembers when galaxies were born, how he liked to stay there, watch the stars begin to burst from blue to yellow until red was all the got left.
He has seen planets being destroyed, and being reborn.
He knows how much time it takes for a grain of sand to be formed.
He witnessed the dawn of man, and all the time it had take them to get to this present. Time as fast as a blink of an eye.
Angels don't blink.
They watch. They guard.
In heaven time passes differently. His time being reeducated was mere days to the Winchesters but to him it was a millenia.
He wouldnt have budged, but he heard the cries of his vessel's blood, felt the danger they were in, a little too late, Dean would say. But he had tried so hard to do the right thing.
Objectively, he knows the time he has spend inside this hospital is infinitesimally shorter than his time in heaven, or even his time on earth. But Lucifer is clever, he knows how to fool him, trick him. Is not a day, but a year, not an hour but eons that pass in front of him.
He sometimes opens his eyes to watch the universe giving birth to the star humans call the Sun; others is to Claire's bright blue eyes reflecting his grace as he entered Jimmy one last time.
Lucifer also knows about his guilt. He knows how to play it, beautiful instrument of torture for Castiel.
And Dean is his masterpiece.
He hears the brother's footsteps before they enter the room. Another trick, perhaps. He was in the ancient Mesopotamia a moment ago, he was on a pier with Dean, fishing, he was trying to tell Dean he shouldn't do that, "there are big plans for that fish, Castiel" someone had said to him once. It fell from his grasp and swam away.
"Hello Dean," —you don't exist,— "Sam" —I'll blink again and you'll be gone.—
He hopes this illusion won't last long; he wishes he could see the drifting of sand at the bottom of the ocean when he closes his eyes again.