Time passing is something that you don't notice until it's no longer there. Doumeki realized this after the first time he fully realized that while he had grown and matured, Watanuki stayed the same.
There was always something about Yuuko that disturbed him slightly, though he could never put his finger on it before it happened to Watanuki. It's a stillness, a quality that's almost frozen, like a photograph or a painting that captures a moment in time. Maybe what conveys "real" to a person is the process of cells dying and being replaced, or perhaps it's something deeper. Whatever it is when it's not there you can tell.
Yet it's not quite right to say that nothing about Watanuki has changed, for there are things that weren't there before. He's gained Yuuko's languid and almost carefree movements, and his speech now has an odd trance-like quality to it. Doumeki doesn't know what's in that pipe and doesn't want to find out, but he is sure that it's more than whatever drug is in there that's causing this.
It's his eyes, mostly. Whenever Watanuki looks at it him it seems that they don't quite focus, and that they're always looking at something just beyond what's actually there. The first time Watanuki turned that gaze on him it scared him, and while it's less shocking now it's no less disturbing. Over time he got used to it, but he can't help but think about the old Watanuki and his clear but stubborn gaze and feel a pang of longing.
Eventually Watanuki grows tired of his company or tomorrow's obligations force Doumeki to leave before it gets too late, and whenever he does he is always struck by the sudden reality of his own mortality as the noise and smell of the city closes in on him. He'll often stop at the gate and glance back at the unchanging shop and its immortal owner.
"How long will you wait, Watanuki?" he mutters to himself, and silently adds And how long will I? before he can stop himself. Eventually he realizes that standing there will do no good and moves on, but not before the desire to rush back in and end this madness grips him like a vice.
But he always moves on, because it's the only thing he can do.