The legal requirement for short term rental is: HDB has a minimum requirement of 6months, and private homes (condo) minimum duration is 3 months.
It's illegal for home owners to rent out a place for anything less than 6 months (for HDB) while private owners are unlikely to want to go through the hassle in settling paperwork for short term leases (private homes has a minimum rental period of 3 months too)
Here's a quick article!
https://www.99.co/blog/singapore/short-term-rental-minimum-3-months/