A Grand Cycle is 17 cycles.
Each cycle is 17 years.
Once every Grand Cycle, the modrons travel around the 17 Outer Planes (clockwise, of course). This is called the Great Modron March, or Modron Procession.
No one knows why they do this, but it's a sure bet they're gathering information.
Source: The Great Modron March, page 4
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