Wikipedia isn't really the most reliable source of information bruffin.
Check out this paper instead. The vaccine has been available in Japan as part of a voluntary vaccination program since 1987 (26 years) and vaccination coverage has been low (around 30%.)
"The low level of vaccine coverage among Japanese infants will not alter the circulation of wild-type VZV, and the epidemiology of natural varicella has not changed greatly since the introduction of the vaccine [4]. In the future, if the rate of vaccine coverage increases owing to the introduction of a compulsory immunization schedule, vaccine recipients will have a reduced risk of being exposed to VZV and, thus, reduced boosting of immunity via natural means. It is expected that a second vaccine dose will be required for maintaining protective immunity against natural disease."
This is what happened in the US. High vaccination coverage meant that there was less exposure to the wild virus and less opportunity for natural 'boosters'. Immunity from the vaccine started to wane and so in 2006 a second booster dose was recommended. So, no, there is not 'no evidence of waning immunity after 30 years of vaccine use in Japan' and the second booster has only been recommended in the US since 2006 so the most we can say the booster lasts for at the moment is 7 years. HTH.