Two times:

1 - Split the balls into three groups of three balls each.

2 - First use of the scale: Weigh any two groups against each other.

3 - If the groups weigh the same then the heavier ball is in the third group, otherwise it is in the group that weighs more.

4 - We now know that the heavy ball is one of three.

5 - Second use of the scale: Using the group of balls that we know contains the heavy ball, weight any two balls against each other.

6 - If one of the balls is heavier then we have our answer. If the balls weigh the same then the third ball is the heavy one and we again have our answer.

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