There is typically a time gap in between when we install a fireplace and when we return to start-up a fireplace. The fireplace is usually unable to work until we assemble the media and do the start-up, which is based on the contractors readiness for that final step. They can hold 10% for which they say is retention or completion. This can take 1 to 2 years until that time. Would our lien deadline be based off of the install date of the return date?
California's mechanics lien is not based on last furnishing. Rather, it's based on overall project completion.
The deadline to file a California mechanics lien is 90 days after the whole project (or "work of improvement") is completed. If a Notice of Completion is filed, that timeframe will be shortened from 90 days to a mere 30 days after completion. More on that here: Should You Monitor for a California Notice of Completion?
So, for a subcontractor, I'd say the lien deadline is based neither on the install date nor the return date. Instead, it'd be based on when the overall project is complete (i.e. when the original contract has been fulfilled and completed, or when some other "completion" even takes place).
For further discussion on CA lien deadlines, here are some good resources: