A 28 year truce

In 1396 King Richard II married Isabella of Valois, the daughter of King Charles VI of France.
Richard signed a 28 year truce with France.
What is the significance of the 28 years?
 
Long enough for Isabella to grow up (because she was still only a child at the time of the marriage) and -- potentially -- bear a child who'd reach legal adulthood themselves?
 
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