What's a working day in an office job?

[Concestor]

This has come up in a work situation where someone gets paid a day rate and logs their hours and I'm curious.

I think a working day in an office job is 7.5 hours, based on the 9-5.30 with a lunch hour. Obviously I know lots of people work different patterns to this these days.

Do you think a working day in an office job is 7.5 hours? Or something else? What hours would you expect from a contractor on a day rate where the hours per day hadn't been clarified/agreed?

I work 7.00-3.30 with 30 min unpaid break. I work in HR in a factory so I start early to service the night shift.
My colleagues do 8.30-5.00.