Thanks for your question. Having previously worked in this environment, I'm happy to share my experience.
I would say it depends on the standards of the country and the company we are supporting. In my case, it was 40 hours a week and the schedule was quite fix since we were working on the field and during planned service closure.
Then depending on the client, they might allow overtime.
When we were in a testing boost, part of the team was alternating to cover night shift of testing. We also supported weekend line closures, which although happening over the weekends, were some of the most interesting part of the testing.
Hope it answers your question! There can be variations based on the country and the company. I would recommend those roles if you like to be on the field, active and ready for the challenge of testing and commissioning.
Thank you for your feedback!