Is there a time limit for the duration of a follow-on task order under a GSA schedule contract for information technology (IT) services and supplies under FAR Subpart 8.4?

The exception states “in the interests of economy and efficiency...”  so it boils down to discretion and deciding are you wasting government’s  and industry’s time with competing knowing which source will likely win?  No, there’s no absolute time limit and you have to use judgement.  It all depends on the exact nature of the requirement and what the incumbent possesses in terms of techniques, skills, methodologies , expertise, and approaches.