Start from the list, not the calendar
Japan's list is currently the shortest in the whole atlas: 14 entries, against an average of 25 and an Italian maximum of 35. That is not a statement about Japan. It is what was left after we purged every templated filler entry from the catalog, and Japan's corpus happened to hold nothing padded. Fourteen entries, every one of them specific enough to plan around, turn out to be a better trip-length calculator than any number of days pulled from a forum.
Look at where those 14 entries actually sit. Five are in Kyoto: the Fushimi Inari trail before sunrise, the Arashiyama bamboo grove early, the moss garden of Saiho-ji, the old streets of Gion, and the torii gates of Fushimi Inari Taisha again as a shrine in its own right. Three sit in and around Tokyo: sushi at Toyosu Market, the Shibuya crossing, and the Shinkansen itself, boarded at Tokyo Station with an ekiben in hand. The rest scatter with intent: Dotonbori in Osaka, Mount Fuji from Lake Kawaguchi, a ryokan night in Hakone, the Hiroshima Peace Memorial, the snow monkeys of Jigokudani in Nagano, and Naoshima's art islands.
Seven days: the corridor
A first week fits the corridor the list draws by itself. Tokyo, Kyoto, Osaka. The Shinkansen covers Tokyo to Kyoto in about two and a quarter hours, which is why the train ride is its own entry rather than a logistics footnote: it is one of the fourteen things, not the tax you pay between them.
Three nights in Tokyo, three in Kyoto, and Osaka as an evening trip from Kyoto for Dotonbori, which is twenty minutes away and best arrived at hungry after dark. That week already touches nine of the fourteen entries without ever feeling like an audit. Kyoto's five alone justify the imbalance: the dawn start at Fushimi Inari and the early walk through Arashiyama are both timing entries, and timing entries need you sleeping nearby.
Ten days: pick one detour
Ten days buys exactly one of the scattered entries, and they are not interchangeable. Hiroshima is the most practical add: about an hour and forty minutes from Kyoto by Shinkansen, doable with one night, and the Peace Memorial deserves the unhurried half day that a day trip from Osaka never quite gives it. Hakone with a ryokan night plus Fuji from Lake Kawaguchi is the other natural pairing, slower and weather-dependent: the mountain hides for days at a time, so treat a clear view as a bonus, not a booking.
What ten days does not buy is both. A schedule that tries Hiroshima and Hakone and the corridor in ten days turns the ryokan night, which exists on the list precisely because it is slow, into a check-in you rush.
Fourteen days: the far entries
Two weeks is when the last three entries become honest options instead of stretch goals. Naoshima wants a full day plus the ferries on either side of it. The snow monkeys at Jigokudani are a Nagano detour that pays best in winter, when the monkeys actually sit in the hot spring instead of beside it. Saiho-ji's moss garden requires applying for a slot ahead of time, which is exactly the kind of constraint that only fits a trip with slack in it.
So the answer the list gives: seven days for the corridor, ten with one detour, fourteen if the far entries are the point. Anchor two or three entries per city and leave the rest liquid, the method from how to use a Top 30 without letting it boss you around. And if you go, come back and vote on what held up. Japan's fourteen are the entries that survived our purge; whether they survive travelers is the part you decide.