Because of wear, not for endorsement purposes. I would guess every couple of tournaments?
How often do pros change their grips?
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I'd guess varies by golfer.
Supposedly Bubba has a tough time when Ping changes his grips. He claims he can detect very minor differences in how the new grips feel compared to his previous ones. He will make Ping regrip each club over and over until they feel right.
Some might change every week or two depending on how much they practice.