Awful. No work/life balance at all.
I was "day" staff but two nights a week, I wasn't getting home until 11pm (after a 7.30am start!) due to being assigned to a house and having to be on duty. I also had to go away on residentials at weekends fairly regularly.
It would only have really worked with a family at my old school if you lived onsite, as some families did.
Lessons officially finished at 5.30 but we were all expected to do at least one "club" or "library" duty and these often lasted until 8.30 pm, with a short dinner break. There were also Saturday lessons.
Then all the open evenings, international obligations, school events (from Speech Night to Chapel to Concerts). Holidays and exeats ARE long BUT there is no guarantee you'll get them all off. Some, mainly international, students stayed at school all year and some staff acted as guardian and there were also lots of summer schools plus the obvious results days (at my school for GCSE, A-Level, BTech and IB!) and insets.
It felt like working at the school meant the school owned you.
There were some senior staff, mainly childless, who lived onsite with essentially free accommodation, bills, food, no travel expenses as they never left school, who had been able to afford several properties outright which they rented out, making them extremely wealthy and able to retire young and for them the pay off was worth it. They lived and breathed school though. I found it very claustrophobic.
I actually walked out in the end after working a 60 hour week on a part-time contract and being refused payment!