Expansion joints

I’m in Minnesota & studying for my jman test & got given practice tests to do. Got told to look through my code books & find answers. I cannot find a definitive answer for the proper lengths for horizontal expansion joints in pvc. I was told by my jman & master that it’s the 35’ rule vertically or horizontally. The book says 30’. I know the book is the final answer, but this rule of 35’, is that an unprinted rule that everyone goes by?

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