Where any party to the intended marriage is not a Singapore citizen or permanent resident, the law requires at least one party to be physically present in Singapore for a period for at least 15 days before the couple can file a notice of marriage with ROM. The 15 days (exclude the day of arrival in Singapore) must be continuous but need not be immediately preceding the date of the notice. It can be in a single visit made to Singapore any time before the notice of the intending marriage is filed.
You may refer to this link to see the available slots for in-house solemnization. The dates shown are for reference only. They are not for booking of solemnizaton slots at ROM.