My secretary for my business made checks payable to herself and deposited them into a bank. The checks did NOT have my signature on them. How long does my bank have to return these checks to her bank?

Under most state versions of the Uniform Commercail Code, your bank has up until midnight of the business day after the checks were presented to return them for forgery, missing signature, etc.
After that time, you may still have a claim against your bank for honoring checks that did not bear your signature. Your bank will do one of two things. They will either accept the loss and reimburse you, or they may deny your claim stating that you were negligent in allowing your secretary access to the checks. Depending on the dollar amount we're talking about, you may need the services of an attorney.