The cycle time is essentially the amount of time a node must wait before old transactions can be discarded. It defines the minimum amount of block history that a node is required to keep. Blocks are said to be "in view" while they are younger than the cycle time and out of view when older. There are several important reasons that nodes must keep a reasonable amount of minimum block history: it ensures that a reasonable size inversion database can be built upon network synchronization, and it helps protect against the secret chain attack.
The cycle time used in Cryptonite is 1 week (10080 blocks at 1 block per minute).