Illinois takes temporary labour seriously, and that changes the sale
Most states leave the relationship between an agency, a worker and a client largely to contract. Illinois does not. Day and temporary labour services here operate under a specific regulatory framework, registered with the Illinois Department of Labor, which places obligations on agencies around notices, worksite information, and how long-serving temporary workers are treated relative to the client's own staff.
Confirm your own obligations with that regulator and your own counsel, because the detail changes and it depends on what your agency does. What matters for the software is narrower and more practical: several of those obligations are triggered by dates, and dates are exactly the kind of thing that a system should be watching rather than a coordinator.
Tenure is a date, so treat it like one
An assignment approaching a sustained duration at the same client is a scheduled commercial conversation. Held early, it is a rate discussion with a client who understands why. Held late, it is a correction, and corrections cost margin and goodwill at the same time.
Putting tenure milestones on the account and generating dated tasks from them removes the only real cause of failure, which is that nobody was counting weeks while they were busy filling orders.