39 CFR § 551.5 - Frequency and other limitations.
(a) The Postal Service is authorized to issue semipostal stamps for a 10-year period beginning on the date on which semipostal stamps are first sold to the public under 39 U.S.C. 416. The Office of Stamp Services will determine the date of commencement of the 10-year period.
(b) The Postal Service will offer only one discretionary semipostal stamp for sale at any given time during the 10-year period, although a discretionary semipostal stamp may be offered for sale at the same time as one or more congressionally mandated semipostal stamps.
(c) The sales period for any given discretionary semipostal stamp is limited to no more than two years, as determined by the Office of Stamp Services.
(d) Prior to or after the issuance of a given discretionary semipostal stamp, the Postal Service may withdraw the semipostal stamp from sale, or to reduce the sales period, if, inter alia:
(1) Its sales or revenue statistics are lower than expected,
(2) The sales or revenue projections are lower than expected, or
(3) The cause or recipient executive agency does not further, or does not comply with, the statutory purposes or requirements of the Semipostal Authorization Act.