How long did ancient Athenian soldiers serve?

The Greek military age was 18 to 60, all of whom were required to turn out in defence of their city. Generally, the age for expeditionary forces was 20 to 45. They were called out as required (and had to turn up with three days rations) and served until the battle or campaign was over, when they returned to their normal occupations, attending periodic training drills.

This applied to people in the cavalry and hoplite (armoured warrior) classes. Those below this were coopted as required as light infantry with bows, javelins and rocks. Later, mercenaries progressively replaced the militias where a leader, city or confederation could afford to pay them.