As a PhD student, I want to limit myself to 40 hrs/week. How to maintain this boundary?
First of all, I know many PhD students (also myself) who did exactly that and finished their phd:
They worked 40 hours a week (or less), had a &quot;normal life&quot; , knew they would go to industry afterwards and wanted to learn/do research before (and stay connected to the system &quot;university&quot;) because they loved uni/studying. It helps that in my country, studying and also titles are traditionally seen as something valuable (so there is no feeling of &quot;only study if this aids you in your future job&quot; in my country).
Some students also saw it as a fun experience to live abroad before returning home.
For me, it was similar: I didn't want to become a researcher because for me the postdoc life seems horrible -- but one can do a phd realtively risk-free. (Now I teach at university).
It is certainly not possible to work only 40h with all profs/in all subjects. Maybe also not in all countries (in which one do you want to study?) Probably it is also not possible with the most famous universities/professors.
I do recommend you to do good research on your prof what kind of person they are. Is it possible to do this kind of work with them or not?
I do think your attitude &quot;I am not dead set on completing&quot; is great. If the prof makes unreasonable demands (or other things like misconduct), just go. When they suggest longer working hours, tell them you don't want to do this unless absolutely necessary, if they keep insisting, just go.
Do keep your eyes open while doing the phd for skills you need in industry.
Note that there are even (incredible) people who finish a  PhD and have little kids (and some of them, no partner!)
(Of course, you might have two fewer papers afterwards for a good university career, but as this doesn't seem to be your goal..)