Hi and welcome to 2CarPros.
A belt should last a lot longer. When you say stretch marks, I can't picture what you mean because if the belt stretches, it is so minimal and happens over a long period of time. As a result, you never see them. If there are marks, I would first confirm that the pulleys are all aligned and the belt is riding centered on each one.
Let me know what you find.
Joe
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Saturday, February 9th, 2019 AT 9:54 PM