Not sure. SQ->SQ is 45 minutes. It will be a lot more than this since SQ and SK are in different terminals. I would guess 75 or 90 minutes.
In any event if you are booking as a single ticket then you wont be allowed to ticket under the minimum connection time, and if booking as separate tickets a minimum connection time is no protection - if your first flight is late then you are on your own.
Baggage handling at SIN is extremely efficient and even the potentially long walk between gates in different terminals can be done airside.