All gross distributions from a Supplementary Retirement Scheme (SRS) account count as taxable income. Qualified distributions are multiplied by 50% before determining the contribution to taxable income.
If you have no other taxable income in that calendar year, then up to $40,000 of gross distributions can be made without Singapore income tax.
Once you start distributions, you have up to 10 years to make them. At the end of the 10th year, anything remaining is deemed distributed and subject to tax. So if you've been pulling out $40K/year for 10 years ($400K) but still have $200K left over, in that last year the distribution (whether you make it or not) would be "deemed" at $240K ($40K withdrawal plus $200K remaining), reduced by 50% (=$120K), and then the $120K would be added to your taxable income for the year. That would trigger some Singapore income tax, obviously.
The only way to extend distributions past the 10 year window is to buy a life annuity in a single premium using SRS funds. NTUC Income and Manulife sell fully SRS qualified single premium life annuities. Those annuities stretch out the distributions over >10 years, and consequently they reduce or eliminate income tax when you have a "big" SRS. You can end up with a "big" SRS if you've saved a lot, had excellent investment results, or some of both. And a big SRS is a happy problem to have, of course.