At least 3 times.

At most, 1000 times or more, if you are unlucky or don't know what to do.

At most 14 times if you are unlucky but you know what to do---weighing two bars at a time.

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- January 18th 2007, 11:02 AMticbol
- January 18th 2007, 11:13 AMCaptainBlack
- January 18th 2007, 11:20 AMticbol
- January 18th 2007, 11:34 AMCaptainBlack
- January 19th 2007, 07:51 AManthmoo
Hahaha! :D

Well AT LEAST once as the question does not ask for a solution but the least number of weighings you could have to determine the lightest one.

This answer is not incorrect and satisfies what the question has asked.

Your answers are all correct as a definite least if you are unlucky all the time - your interpretation but not what the question asks.