IRS Letter 4464C

Client received IRS letter 4464C which seems to be a random review of her 2017 tax return for no specific purpose. The return has nothing particular controversial or unusual, although her refund is larger than usual due to a number of several routine situations. It appears this review serves primarily to delay her refund for 60 additional days.
Does anyone have any experience with these letters and how long this takes to resolve?
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