Depends on what you are interested in, I suppose.
Allow one full day to sight see. Consider taking a rickshaw tour of the downtown area, stopping for a Singapore Sling at the Raffles Hotel and a Bug-Bug boat trip along the river.
You can spend a day shopping but I seldom find bargains in Singapore apart from electrical equipment, so if you are after a laptop computer or two....
The charm about Singapore is the variety of exotic food around - Chinese, Malay, Indian, Thai and various combinations in-between so give MacDonalds and KFC a miss (even though you will see a few around).
3 days is plenty.