|This is just my opinion, Since I love Detective Conan i made a comment hehehe
It seems to be it couldn't be more than a year,
Spoiler (mouse over to view)
During the eisuke/kir-arc alone there are some serious and problematic time discrepancies... you may remember the case with the dead nail-snake singer.. this took place like a week after new-years... some cases later in the "clash of red and black arc" climaxing in akai's death eisuke vanished on the 18th or 19th or so of december (which is at least 2 weeks earlier)... so logically speaking there should have passed a year but it didn't..
So in the manga, sometimes cases occurred takes back a season (like how many valentines and Christmas event happen) is the point is -- not chronological when it comes to TIME and not to mention they also do flashbacks (or event happened in the past) chapters. Another thing, considered that the story does taken into parts (part 1,2,3...etc) which occurs like 2-3days depending on the case they have.. but hey this is a manga time goes VERY slow
And Gosho doesn't mind as long as the main story is chronological lol.. oh yeah another thing imo (somewhere in the manga) i think Ran moved a semester o_O