I think it depends on the individual person. I have seen some people say they charge by the hour and others have said they charge depending on the specific job. I think it depends what you are most comfortable with.
I have no clue how anyone can bid a job and not think about how long it will take.
You say you don't charge by the hour but you do you just don't bid it per hour you bid one set price but you came to this price by using how long it will take. So yes by the hour
(( cowboy up and get over it ))