We rent equipment for large poured in place concrete projects. The projects can last 6-12 months. I have been told the timeline to file a lien starts 8 months after the date of last shipment of equipment. However, since it is rental equipment and we retain ownership of the equipment and the potential length of the project, it seems the timeline to file a lien should start 8 months after the last return of equipment.
It would depend on the terms of your contract. If your rental changes continue to run through the date the last equipment is returned, I think you're safe in calling that your last day of "work" for purposes of computing the deadline to file the lien.