If we are to believe the stories told in Legends, the first Guard Mouse came after the line of the kings, both of the Silver Crown and the Golden Crown.
So two generations after the king who died in 942… But kings often die relatively young, so who knows when.
As David Petersen did draw and tell the story behind the cover of the Legend of the two Kings who faught eachother in 942,
that feels canon and so des the Legend about the last Silver Crown king who was the grandson of one of them.
That means the Guard is only between 150 and 200 years… But I know, it might just be legends, not canon at all, even if the mice in June Alley Inn should know their history! 
But does anyone know? Did I miss it somewhere?