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I request all the time, for no reason really. I was probably requesting once a month for a while when it was around 10k, always got a no, but then one day, random email came saying they were upping it to 17.5. I forget when they moved it further than that. But I don't think I've ever gotten a "yes" from clicking that button, and I've done it a LOT. Always get "not enough usage." But I also don't think clicking the button all the time hurt me too much since they've given me credit increases still.
This is my highest usage card by far, but it's usually in the 700-1200 dollar range, not anywhere near my limit, so I don't blame them.