How long has it been now? The thing about being permanently trapped and unable to move or speak as a part of another living creature is that after a while you stop being able to keep track of how long its been since you still had a life. In your specific situation, however, there is a way that you can keep track of the days, although after a while it's less painful to just stop keeping track. You don't know much about dragon biology, but you assume he defecates once a day? At least that's approximately what it feels like to you. You don't see any sunlight down in this cave, so you could be way off. Going by this hellish new unit of time that embodies the disgusting misery your existence has become, by your count you estimate that you've been trapped in this hell for... two hundred and fifty... seven, you want to say? Two hundred and fifty seven days. You'll just go with that number. Not like it matters, it'll only keep going up.
Over two-thirds of a year. And he said that dragons on average live to around a thousand. You've read about "fates worse than death" in fiction over the years, and usually the victim eventually goes insane. Loses their grip on reality and drifts into a world of their mind's making. True, you mostly exist in your own thoughts now, but your senses are all still perfectly connected. Those words he said to you after your very first experience doing an anuses' job still haunt you. "I've made sure that the magic permanently binding you to my body will keep your mind intact for the rest of my existence." As much as you want to, you will never escape being nothing but an anus. You are locked into this form; you will always be directly plugged into your senses. You will never, even for a second, be able to forget or pretend that this isn't all that you are now.
You've begun telling yourself stories to keep your mind busy, to keep your mind off your life as much as you can. You write books in your mind. It doesn't quite distract you from the shape of your body, the constant stench, the permanent taste. But at least it's something to do. One of the only things you can do now.
The dragon gets up from his slumber. You know what time of day it is. As many times as you've been subjected to this already, you still at least try to fight it. With all of your thoughts you tell your body to do something, to resist. Nothing happens. You don't move. You have no control. Your anus body doesn't even so much as twitch. All you can do is watch and wait as the dragon walks slowly over to another part of the cave, digs a small hole in the right spot, and squats. It never seems to get any easier: the stretching out, the feeling of being intensely squeezed, the sensation of each and every subtle contour of the massive turd. And of course, the taste. You don't know what kind of sick monster designed this magic, but whoever they were, they saw fit to make your entire anus body able to taste like a tongue. As many times as you've been forced through this, it never gets any better or more bearable. The worst part, at least you think, is how completely unable to react you are. There may be a human mind occupying this small piece of dragon flesh, but an outside observer would never know. You can't express how much you desperately want to throw up. You can't scream. You can't cry. At least never externally.
The dragon returns to his favorite spot, curls up, and falls back asleep. Two hundred and fifty-eight days into year one, now. Only about eight hundred years left.