While it doesn't have specific retention periods for various forms, the Code of Federal Regulations, Title 29, Part 516 (Records to be Kept by Employers), U.S. Dept. of Labor, Employment Standards Administration, Wage & Hour Division, has some guidelines for types of records (if you can wade through all the legalese). My copy is old, but you should be able to get an up-to-date copy from the Gov't Printing Office (or maybe it's even online now).
Good luck!