Route Bid After 204b

How long can a city letter carrier work as a 204b, higher level assignment, before their route goes up for bid?

A city letter carrier can work as a 204b, or in a higher-level assignment, for up to four months before their route must be declared vacant and posted for bid. If a carrier is detailed to a supervisory position, including a supervisory training program, for more than four months, the duty assignment shall be posted for bid in accordance with the National Agreement. Upon returning to their craft, the carrier will become an unassigned regular【12†source】.