Since you are doing ducts, pipes and large walls, can I assume you are using spray equipmnet, and have been working for a painter ?
Most use prior jobs as a guess to what new jobs take.
So from your prior painting experience you will have to guess how long it will take to do the job you are looking at.
If you have no commericial experience perhaps you should considered working for a painter for a year and learning everything you need to know to start.
Also do you have insurance, esp liablity insurance, if you get paint on their screen printing machines, or on the products they are doing.
If you are hiring anyone, do you have workers comp insurance, many states require you have insurance on any employees.