We spent 6 days in Singapore last November (had planned on 3-4, pre-cruise, but because we wanted business class award tickets, we took what we could - I ended up working/visiting a collaborator in SIN one day, so that was nice - 1 fewer vacation day I needed to use)

6 days was more than we needed, certainly. But it was very nice to spend relaxing days doing one or two things and getting over jet lag in a gentle way, having an afternoon drink at the hotel pool.

The heat & humidity though. Wow. We live in North Carolina - and unlike many people, actually spend time outside in the summer. I thought I understood and was prepared for humidity. I was WRONG.

On the other hand, it is VERY easy to go inside to air conditioning and/or get a cool beverage whenever needed.

We got free HOHO tickets from our hotel (Oasia Novena - loved it) so did that the first day - to prioritize our list of things we wanted to see in more detail.

To answer the OP question from the point of view of a new tourist to Singapore, I would vote 3 days (I am also NOT a beach person, so an extra day on a beach in Bali would not appeal - so that's my personal bias)