The Great Cycle is perhaps the most fundamental law of nature on Dragaera.
A House's position on the cycle also affects how well fate favors each house. This is somewhat similar to the phases of the moon--a House whose position is rising in the Cycle, such as the Dragon, is gaining favor, while the Athyra whose Reign has passed "recently" will continue to lose favor until the next Vallista Reign, when it will start to rise again.
The time a cycle takes to complete depends on the length of each Reign. A Reign may be as short as 289 years, or as long as 4,913 years. It may be possible for several different emperors of a given house to rule during a single Reign, but once a Reign ends, leadership of The Empire passes to a new House.
Thus, a single Cycle could be as short as 4,913 years, or as long as 83,521 years.
Seventeen Cycles make up a Great Cycle. The Empire being well over 200,000 years old, we are now just barely into the second Great Cycle.
The Physical Cycle
The Cycle is not just a mental construct but is an actual object located near the Halls of Judgment in the Paths of the Dead. It is a massive stone wheel, mounted on the side of a great wall. According to Dragaerans who are authorities on the subject, this artifact is not simply a manifestation or symbol of the cycle--this massive stone wheel actually is the cycle in every important respect.
It is said that if someone were able to physically turn the stone wheel, then the cycle itself would, in fact, change. Given the mass of the cycle, this does not seem likely. It should be noted that the cycle apparently rotates in a counter-clockwise direction.
An image of the cycle can be found here.
Copied from Mindstalk's Steven Brust fan page The poem can also be found in the beginning of some Vlad books.
Note: Not just a poem.
Phoenix sinks into decay,
Haughty dragon yearns to slay.
Lyorn growls and lowers horn,
Tiassa dreams and plots are born.
Hawk looks down from lofty flight,
Dzur stalks and blends with night.
Issola strikes from courtly bow,
Tsalmoth maintains though none knows how.
Vallista rends and then rebuilds,
Jhereg feeds on others' kills.
Quiet iorich won't forget,
Sly chreotha weaves his net.
Yendi coils and strikes, unseen,
Orca circles, hard and lean.
Frightened teckla hides in grass,
Jhegaala shifts as moments pass.
Athyra rules minds' interplay,
Phoenix rise from ashes, gray.
- See also Speculation:Cycle.