5 CFR 334.104: Length of assignment.

(a) The head of a Federal agency, or his or her designee, may make an assignment for up to 2 years, which may be extended for up to 2 more years if the parties agree.

(b) A Federal agency may not send an employee on an assignment if that person is a Federal employee and has participated in this program for more than a total of 6 years during his or her Federal career. OPM may waive this restriction upon the written request of the agency head, or his or her designee.

(c) A Federal agency may not send or receive an employee on an assignment if the employee has participated in this program for 4 continuous years without at least a 12-month return to duty with the organization from which the employee was originally assigned. Successive assignments with a break of no more than 60 calendar days will be regarded as continuous service under the mobility authority.