I am not aware that there is one. It would likely fall under employment and contract law. What is "reasonable"?
If you enter into a contract, be sure you understand and are happy with the terms contained within it, before you sign.
If you pay for a type rating yourself, you can enter into whatever loan terms and duration, that you and the lender agree. That might be one year, two years, three years, etc.
If the potential employer is by way of contract providing you with the tools necessary for your employment (in this case a type rating) they can amortize the effective loan (bond) over whatever time scale you and they agree to, provided that the terms are reasonable in the circumstances, and that you can satisfy the contract at any time by paying off the balance owed.