I think you need to say "usually" rather than "often."
"Often" means "frequently," but that's not what you want to ask, is it? You want to ask about the average length of the lunch break. Or even the fixed length, if it is fixed. In either case, "usually" works better.
Yes, you can do, but then you tend to exclude the possibility of flexible working hours. Some companies have a fixed-length lunch break, other allow their staff to work and stop more or less when they please.
Your alternative sentence suggests that you are expecting to hear the length of a fixed lunch break.