When is it too late for borrower to withdraw loan application?

You can withdraw a loan application anytime up to the funding date of the loan (on a purchase, the day of closing, and on an owner occupied refi, the fourth business day after closing). Bear in mind that any appraisal, application, credit, or rate lock fees you have paid will likely not be refundable, but if you have discovered the terms of your loan are unacceptable or far worse than those you could get elsewhere, probably better to withdraw rather than close a bad loan.