Page "Peloponnesian War" Paragraph 16

The Spartans also occupied Attica for periods of only three weeks at a time ; in the tradition of earlier hoplite warfare the soldiers were expected to go home to participate in the harvest.

Moreover, Spartan slaves, known as helots, needed to be kept under control, and could not be left unsupervised for long periods of time.

The longest Spartan invasion, in 430 BC, lasted just forty days.