Cycle

A cycle is a measure of time used by orcs. Young orcs do not get named until the Naming Day of the cycle they were born in.

Azeroth's moons
As Thrall was about to attack an internment camp, one of his men informs him that both moons are full and that maybe they should attack when it is darker. Thrall states that they cannot wait two more weeks to attack. It would seem that two weeks elapse between a full moon and a new moon.

Speculation
A cycle may be the equivalent of a year for orcs. However, they never used it in that context on Draenor or Azeroth, simply referring to years as years. It is more likely a reference to a lunar cycle, which goes through noticeable phases.