On a gross basis, the Bank purchases JGBs at an  BIS central bankers’ speeches  7  annual pace of about 110 to 120 trillion yen, which is equivalent to about 90 percent of the total amount of JGBs’ market issuance.
If final investors were to reduce their holdings of JGBs by redeeming the bonds each time they come to maturity and not reinvesting the amount redeemed in JGBs, the Bank’s massive JGB purchases would be sustainable. However, in reality, there is a certain level of demand for JGBs, mainly for those used as collateral.