(1)  Put in freezer:  20-25 minutes

(2)  Put in bucket of ice: 10-12 minutes

(3)  Put in bucket of ice and water:  4-6 minutes

(4)  Put in bucket of ice and water and salt:  just over 2 minutes

(5)  Put in bucket of ice and water and salt, while rolling can around:  Less than two minutes

(6)  Fastest way possible: blast the soda with a CO2 fire extinguisher

So says one of the guys from Mythbusters