Leto's Peace lasted approximately 3,500 years. It meant that none of the Empire's stakeholders could go to war with one another. This was accomplished by maintaining strict control over space travel, and forcing peoples to remain planet-bound. It was part of Leto's Golden Path, and was a contributing factor to the Famine Times and the Scattering after Leto's death.
Leto's Peace had been developed into what it was from the Guild Peace that had been in place for more than 10,000 years prior to Leto's reign.