The question then becomes for freelancers, when you’re in the middle of a longish contract, how do you actually maximize your billable time? Eg do you find client work that much more palatable that you actually do manage to work 8hrs a day on it? Are your rates high enough that you can do about 4hrs/day and still come out ahead? (Probably took a long time to get to that point?) Something else?
On the other hand i often negotiate a flat fee for a specific task... For example my last one was solving a very specific computer vision problem for a client after their inhouse project team(s) of 5 people were not able to solve it over the course of 6 months.
It took me about 8 hours in total to solve it, and maybe 40 hours to integrate it with their product/document it.
However i billed them close to 100.000 € for that task, ( pre agreed by them upon successful completion ) and worked bascially a week only.
But compared to 5 people * 6 months * salaries of 100k = 250.000 it was a bargain for them ( not counting overhead costs and the likely hood of further time not solving it )