How long is the Japanese visa extension at most?

There are many kinds of visas and the time is different. The longest tourist visa is 15 days.

If you use a tourist visa to study in a language school in Japan, or have relatives in Japan, it can be extended to 1-3 months according to the situation. The longest is half a year, and you can reapply for three months after three months, but it depends.

For example, under special circumstances, such as giving birth to a child or having a serious illness, the change of visa application in the middle is generally not more than one month.

Extended data:

Visa is the annotation and seal of the sovereign organ of a country on the passport or other travel documents held by its own or foreign citizens to show that they are allowed to enter or leave the country or pass through it. It can also be said that it is an endorsement certificate issued to them.

Generally speaking, a visa is a document issued by a country's entry-exit administration (such as immigration bureau or its embassy or consulate abroad) to approve the entry of foreign citizens.

References:

Japan Visa-Baidu Encyclopedia