You can choose whether you wish to follow a short-term or a long-term program. The classic short-term programme in Japan is during the summer, which is convenient since you’ll usually be able to stay in Japan on a student visa. Few schools offer short-term programmes with a specific focus, such as business Japanese or conversation, but rather the classes are divided up according to level. If your ultimate aim is to get your Japanese to a good enough level to enter a Japanese university or work in a Japanese language environment then you should be looking at a long-term study programme of a year. Long-term study will require a student visa.