I expect a high degree of cultural variation here. To illustrate: In the Netherlands 25% of the male workforce and 75% of the female workforce was working parttime (<35hrs/wk) in 2009. I don't have more recent figures ready, but the Netherlands has passed the 'Law on flexible work' in 2018.
It has thus become a question of cultural fit. In the Netherlands, parttime work is more prevalent in smaller companies with strong family values. And (semi-)government with strong unions typically has a 36- or 38-hour default contract size. Employers understand that a good work-life balance translates to more motivated, productive and loyal employees.
From personal experience: Most jobs in the Dutch software industry are listed as full-time (5x8=40hrs/wk, excluding lunch and coffee breaks), but a minor reduction in contract hours (to 36 or 32hrs/wk) is virtually always negotiable. Bigger reductions than that are more difficult.
In your locale, this may be quite different. But there will be companies that are open to the discussion. Look for positions that mention flexible work hours ("core hours 10-15"), the option of working from home and similar perks.
If a full-time work week is an absolute dealbreaker to you, I (as an interviewer) would prefer you to discuss it near the end of the first interview. I like candidates that leave me with the following impression:
- That it is your preferred balance between the employee you want to be and the father/craftsman/sportsman/community member you want to be. You have a specific thing that you want to spend the time on.
- That you are determined to excel in both, without your after-work activities influencing your productivity. "Work hard, play hard" is a tired cliche, but I understand it viscerally.
- That you are willing to tailor your needs to those of the company. (Fridays off? If we schedule an important deployment, would you pick a different day? Would you do 36hrs instead of 32?)
- If you signal the intent to establish a long-term bond with the company. Kids in school? Family nearby? Looking for a house or living in the area? Long tenure at previous jobs?