Twice
Strategy 1.
1) Make 2 groups of 4, measure, 2) make 2 groups of 2 from the heavier group, measure, 3) measure the heavier group 1+1 to identify the heaviest one.
4+4 ---> 2+2 ---> 1+1 , total 3 measurements
Strategy 2.
1) Make 3 groups of 3/3/2, measure 3 and 3, If they are equal, 2.1) measure 1+1 and find heavy one, if one group of 3 is heavier on first measurement, then, 2.2) divide the heavier group into 3 and measure 1+1, if any one is heavier, it is found, if they are equal then the reminder in group is the heaviest one.
3+3+2 ---> 1+1, total 2 measurements
As you can see the best is the strategy 2.