Translation:How many kilometers do you run per week?
I answered 'How many kilometres do you run per week?' and it said it was incorrect, because I used the correct English spelling of 'kilometre'. Need to update the right answers on this one.
For a week must be a better translation since it does not imply a specific week. For is not accepted though.
"For a week" is not good American English in this case. Better translations are "per week", "in a week", "each week". All of these imply that you are asking "How many kilometers do you (normally/usually) run per week?