How exactly is the withdrawal period determined at retirement?
The withdrawal period starts when you make your first withdrawal at or after the statutory retirement age that was prevailing when you made your first SRS contribution. It will end 10 years from this date.
However, the 10-year period does not apply to investments in life annuities. So long as you continue to receive your annuity streams in perpetuity, the 50% tax concession will apply.