14 C.F.R. § 262.8
A carrier shall not impose unreasonable restrictions, conditions or limitations on the travel credits or vouchers, including a validity period that is shorter than five years from the date of issuance, a restriction on the transferability of the credits or vouchers to another individual, conditions that severely restrict booking with respect to travel date, time, route, or class of service; a limitation that allows redemption only in one booking and renders any residual value void; or a limitation that only allows the value of the credits or vouchers to apply to the base fare of a new booking but not government-imposed taxes or fees, carrier imposed fees, or ancillary service fees. A carrier must clearly disclose, no later than at the time of voucher or credit issuance, any material restrictions, limitations, or conditions on the use of the credits and vouchers that are not deemed unreasonable, including but not limited to advance purchase requirement or capacity restrictions and blackout dates.
14 C.F.R. §262.8