Stopover, city break or a month
A stopover of two or three nights, or a long weekend of Gardens by the Bay, Chinatown and an afternoon on Sentosa, uses little data: maps, Grab, the odd video call. The 3 GB pack, valid for 15 days, covers it with room to spare, and because the plan starts at the first connection in Singapore, you can buy it weeks ahead.
A week or more, or a work trip where you tether a laptop between meetings, is a 5 GB trip, and that pack runs for 30 days. Take 10 GB if you stream in the evenings or share the connection with someone travelling with you, and 20 GB if you are staying a month. Flying on afterwards? The Indonesia, Thailand and Vietnam eSIMs are separate packs. The data calculator sizes it for your own plans.