I lived in SGP for a while some years back!
6 hours. Then it you have time, but not that much! I suppose that it depeds on the time of the day.
The typical touristy thing to do is to go to Raffles Hotel and have a Singapore Sling in the Long Bar.
For us it would be enough time to go to East Coast Seafood Center and get some Chilli Crab.
East Coast Seafood Center is close to the Airport and very easy with Taxis.
I am not sure if that Pool & Poolbar on the roof of one of the Terminal ( inside the Airport) is still there!