This varies.

Some places do a 37.5 hour work week, 9-5 with a 30 minute lunch. Others do 9-6 with an hour. There's also no shortage of places (such as my workplace) that do 35 hour work week, 9-5 with 1 hour lunch.

Well, assuming you work for a company that puts much stock in ass-in-seat time (which, lucky for me, I don't).