In the writing, I was doing recently, a character is locked up in a wagon and dragged across the country by rich slavers. How could I show the reader that time is passing when the character wakes up only occasionally, seeing very little of the outside world, and is drugged to unconsciousness whenever they are seen awake.
The character is in a caravan with many slavers and three other prisoners - two girls unknown to him, and a childhood friend he was captured with.
What sort of time period are we looking at? Hours? Days? Weeks? Months?
Over a period of hours, it's going to be things like the angle of shadows (how high in the sky the sun is), the temperature, et cetera.
At days, it comes down to water breaks: a dead slave is worth no money, so they need to be given something to drink occasionally. Are the drugged slaves roused from their stupor just enough to manage this without drowning?
Once you reach weeks, you start to notice things like beards growing, or weight loss from not being fed properly. Add food to the water breaks - probably a gruel of some sort, not particularly nice or nutritious, but still an indication that time has passed.
Consider also the Slavers themselves - the guards in the caravan will rotate through a shift schedule, they will (presumably) change clothes every so often. And, finally, geography - different sounds and smells may indicate to the character that they must be at least such and such a distance from where they started. Brine for sea ports, certain types of flowers or animals, the quality of the road or track that they are travelling on, the accents of people talking outside the caravan.
Changes of light, of temperature, of weather, of season. Unremembered wear in clothing, the healing of wounds, loss of physical condition. Hair longer than ever before. Sudden brutal grooming from the captors and subsequent regrowth. Being let out to push a wagon out of mud. Sickness in the camp. Raiders, fended off. Capture by the raiders, and subsequent recapture by the slavers. Events.
It's a wagon, not Schrodinger's Catbox.
I think George R. R. Martin does this quite well in his various novels. A good few characters in A Song of Ice and Fire are locked up at some point, and GRRM always dedicated a good couple of pages at least to hammering home how long they've been in captivity.
These chapters often deal with the mental toll of being confined to a place with no brain stimulation, just as much as the physical toll of wasting away. At the start of such a chapter, the character attempts to keep track of days by counting how often they're fed. Inevitably they lose count as the days start to blend together. They try to keep their mind occupied by playing games with themselves or by trying to recount old stories, but that inevitably stops after a while, and they find themselves sleeping most of the time. At some point, their dreams and waking become indistinguishable and they feel like they've been in captivity for decades even if the actual time spent is only a couple of months.
Your character's captivity will take a toll on him, both physically and mentally.
In addition to @Chronocidal's answer, you can also use nature to show the passage of time.
Even within the confines of a wagon with no outside view, you could have mice that are breeding. They can have up to 10 litters a year, so plenty of generations to be born, grow up and leave.
And if you can cast a glance outside through a slit in the wagon or something, you could see birds making nests, spring/summer/autumn plants starting to flower, trees getting or loosing leaves, dry/rainy seasons etc.
And on even longer time scales, if the group passes the same place multiple times, you can use things like trees growing and falling over, abandoned pastures slowly getting covered by forest, fires clearing a grassland/savanna and subsequent new growth coming back, etc.
Basically at every time scale you can find things in nature to change, because nature has generational cycles ranging from days, to centuries. Nature may not be the easiest to use for the shortest time scales though since they mostly cover small insects, fungi, etc that most people wouldn't normally notice.
If it were a film, you might likely see a montage of the prisoner waking and each time either falling back into a stupor, being drugged, or beaten, or fed. They might get progressively more dirty and unkempt. You might see this overlaid with the wagon constantly riding through the countryside.
In written medium you must convey this explicitly but you can.
The prisoner woke. He slept. He woke. He slept. Night followed day, day followed night. At intervals the wagon stopped and he was roughly shaken awake to eat mouldy bread and drink rancid water. Every 6 or 7 stoppages, a bucket of the same water was thrown over the prisoners to wash off the worst of their filth, and the other, unmentionably-filled buckets were removed and emptied.
Are you using first-person perspective or do just want to show instead of telling? Do you want the character to be aware of the time passing or just the reader? Do they need to notice time passing as it passes or would it be an option to have the character realize sometime after the arrival that the time has passed?
If it is first-person and you want to credibly convey that the captive is drugged, it might be more credible to only have them realize later how long they were in that waggon.
If your character is a man (in the sense "not a boy anymore"), you could use his beard growing as an indication of time passing.
You might like to use dialogue between the slavers to imply the passage of time. They might reference how long the journey has been going, how long is left, meal-times, etc.
Dialogue can be a natural way to push information you want the reader to have.
