If/when organizations send a letter to commit to setting a science-based target, they will then have 24 months to submit their targets to the SBTi. For example, if a financial institution sent a signed commitment letter in November 2023, then its target submission deadline would be in November 2025.
For financial institutions with target submission deadlines up to six months after the FINZ Standard publication, this extension option that you referred to is available to them.
You can find more information (including tentative timelines) in this announcement calling for pilot test participants for the FINZ Standard draft.