RetireJapan

How long does one have to be in Shigaku kyosai for the pension contribution part to translate into some kind of pension?

120 months / 10 years of payments are needed for Japanese pensions, to receive anything.
This is general info, not specific to shigaku kyousai.

To confirm %, I suggest calling their pension section:

年金部年金第一課
電話：03-3813-5321（代表）

If you become a permanent resident your years abroad count towards the 10 year minimum.

Thanks to you both for this helpful info.