David was boss for 7 years and 6 months in Hebron town. (We have not yet translated 2:12-32. It is about a fight between David’s mob and the mob that wanted Saul’s son to be their boss.)
Ishbosheth was 40 years old at the time he became boss over those Israel tribes, and he was their boss for 2 years. But the Judah tribe made David their boss.