Singapore HDB resale: how long is the Option to Purchase valid?

For an HDB resale flat, the official Option to Purchase is valid for 21 calendar days from the date it is granted. The buyer can exercise the option within that period or allow it to expire. HDB says the OTP is a legally binding contract and buyers should confirm financing and value before exercising it. Official source: HDB. Guide: https://getinstantlaw.com/property-buying-selling/contract-problem