5 CFR §334.104

1. (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.
2. (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.
3. (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.