My first unpaid six-hour take-home assignment. At what point does 'assessment' become free labor?

Take-home 'assessment' for a mid-size company: build a working feature with tests and docs, estimated 2-3 hours. Took me six. Found out afterward they'd sent the same assignment to 40+ candidates. That's a lot of free, unpaid product work if none of it goes anywhere. At what point does an assessment stop being an assessment?