I didn't see anything in the MBA retention book...although there is some quasi-related language about fair housing in the mortgage section that seems to point towards 3 years. (See "Mortgages - Fair Housing Laws Compliance" on page 63.)
At a minimum, I'd retain from fair lending exam to fair lending exam. If the examination is clean, I'd probably purge anything older than 3 years.
_________________________
Opinions are Bartman's, not those of my employer. "A noble spirit embiggens the smallest man."