Over the next week we are going to see a lot of debate about the substantive scope of the Syria/WMD AUMF, much of it directed at whether the administration's draft or other draft language might end up being cited as justification for uses of force beyond what the administration has clearly stated it has in mind. Whatever the final language turns out to be, some version of this concern is likely to remain. Bearing that in mind...why not a sunset clause?
Update: For the record, this (or at least something akin to this) has been done before. See, for example, section 6 of the statute authorizing US military participation in the Multinational Force in Lebanon in the early 1980s:
DURATION OF AUTHORIZATION FOR UNITED STATES PARTICIPATION IN THE MULTINATIONAL FORCE IN LEBANON
SEC. 6. The participation of United States Armed Forces in the Multinational Force in Lebanon shall be authorized for purposes of the War Powers Resolution until the end of the eighteen-month period beginning on the date of enactment of this resolution unless the Congress extends such authorization, except that such authorization shall terminate sooner upon the occurrence of any one of the following:
(1) the withdrawal of all foreign forces from Lebanon, unless the President determines and certifies to the Congress that continued United States Armed Forces participation in the Multinational Force in Lebanon is required after such withdrawal in order to accomplish the purposes specified in the September 25, 1982, exchange of letters providing for the establishment of the Multinational Force in Lebanon; or
(2) the assumption by the United Nations or the Government of Lebanon of the responsibilities of the Multinational Force in Lebanon; or
(3) the implementation of other effective security arrangements in the area; or
(4) the withdrawal of all other countries from participation in the Multinational Force in Lebanon.