How long does it take to visit the entire Japan? How much? What suggestions do you guys have?

Only 15 days is not enough to visit the main cities of Honshu Island: Tokyo, Osaka, Kyoto, Nara, Kobe, and Nagoya. Hokkaido takes 7 days. For Kyushu, Fukuoka, Nagasaki, Kumamoto, and Kagoshima It takes about 10 days to visit cities like Miyazaki, 5 days for Shikoku Island, and 3 days for Okinawa, Ryukyu. You can visit all the major cities in Japan in 40 days. The cost is 600 a day for accommodation, 400 for food, and 400 for fun. 800~900, leave more than 100 for transportation, 2,000 per day, 80,000 per day, divided into 5 tours, each round trip ticket costs 4,000, 20,000 per day, it takes 40 days to visit Japan, 100,000 yuan, So you can choose the city you like. I highly recommend cities such as Tokyo, Osaka, Kyoto, Nara, Nagoya and Okinawa. Try to use the remaining air tickets at one time. If you buy the air tickets a year in advance, you can go back and forth to Osaka for only about 1,400. Then depart from Osaka. Osaka is recommended. Universal Studios, Tokyo recommends Disney, Okinawa is more unique, Tokyo is more shopping, Osaka is more fun, Okinawa is more sea view, you can choose from any of the major cities recommended above