It's 23 days according to[1] which in an unheated (or conversely uncooled) cell along with "confess even if you didn't do it, it will make it easier" spiel can be hard to withstand.
My bad, I misremembered, the base is actually 3 days (72 hours, that's the number I actually knew, but I mixed up with something else), and can be prolonged for 10 days by prosecution if a judge allows it, and can be prolonged for 10 more days after that, again validated by a judge. After that, the detainee needs to be either charged or released.
Supposedly, the extensions are for cases where there is risk of destruction of evidence or disappearance. Wikipedia cites some (old, 1987) numbers that 85% of arrested detainees were kept for longer than 72 hours and more than a third were held without charge for longer than ten days.
Call me skeptical regarding being held without charge in Japan. I know of a certain infamous French national that was held for months without charge there. [1]. The loophole I guess was adding a second charge when the first 'grace period' expired.
[1]http://japan.embassy.gov.au/tkyo/arrests.html