How much time has passed in-universe in Detective Conan?
Technically the story plays out in real time, but realistically it is at the whim of the author.

Assuming the cases appear in chronological order. You'll notice that none of the characters ever age or even graduate. There are many discrepancies the Eisuke/Kir Arc alone has some serious and problematic time discrepancies. It starts with the Nail and Snake Arc which took place a week or so after New Years and then continues to some cases later in the Clash of Red and Black arc which climaxing in a certain person's death. Eisuke vanished around 18th to 19th of December... so chronologically speaking a year should have passed... but did it? Ask Gosho Aoyama.