Six hours
After arrival immigration and before departure security you have about three. Jewel is the realistic outing, or a transit hotel block if you are staying airside.
Twelve hours
A day cabin at YOTELAIR from four hours, or a transit hotel block. If it is daylight and your passport allows entry, a few hours in the city works, with the train and a firm return time.
Twenty-four hours
Enough for a proper hotel in the city and an evening out. Collect your bags if they are not through-checked.