when writing a story set in a fantasy world, or reading, how do you do common knowledge stuff like measuring time, or days of the week. or distance. and how much gold/silver/copper/seashells does it take to buy a horse anyways? I have seen it done a lot of ways, but I find that my friends and I differ a lot with how we like to read it. for example I would be completely thrown off if some arcane text told the wizard hhe would need to open the portal to the demon plane TUESDAY after next(unless humor was the object) while at the same time I hate it when armies march for ten leagues, because leagues is so over used in actual distances I never know if its a mile or three or something completely different. and if you do create something different, how should you catch the reader up without making the characteers discuss plain knowledge? any thoughts
Have characters there to whom it is logical and appropriate for some of this stuff to be explained. Foreigners, children, and people from other worlds show up in fantasy a lot because they make this easy.
in a first-person narrative, you have the option of the narrator explaining things directly, though it's much harder and more fun to slip the things in so they are obvious as needed.
I don't, myself, favour the long infodump unless you happen to be Neal Stephenson, which very few people are. Cluing things in can be a pain, though. I have one story setting which is a quasi-medieval tech-level lost colony on a planet around a blue-white sun. I've done the sums to figure out how hot the blue-white sun is, and how far out the planet needs to be in order to be habitable, so their year is two and a bit Earth years; this is of course not a comparison they generally have, but one way of getting that across in conversation was a character realising she was pregnant and thinking what time of year the baby would be born, combined with it being clear what time of year it was when the scene was actually happening.
Units of measurement tend to come from fairly basic human things that people have from way back in time. A mile comes from the Latin word for "thousand" meaning a thousand paces, for example. I have considered "mansheights" as a unit; the thing is, words that get used a lot tend to be abbreviated over time if they are not short and snappy, and two syllables is as much as you can reasonably expect to see used colloquially IMO, which does not leave you that much space to make the derivation clear.
On the other hand, you can just snark about it. There's a lovely line in one of Steven Brust's fantasy novels to the effect that "Fortnight was an Eastern word, though why they needed a word for a chunk of time one day shorter than three weeks is beyond me"; which tells you that either fortnights or weeks in this world are not what you expect them to be.
