Why not mnatembea kwa saa ngapi? I selected both options as from the English you cannot tell if it is you singular or you plural.
Yeah, just report it ... this is a problem throughout the course.
Actually ... I just remembered that you can't report it anymore because it's only one letter wrong so the site marks it as a typo and doesn't give you any options to report the English.
Would "unatembea masaa mangapi" not be more appropriate
In English we wouldn't say "For"... We would just say "How many hours do you walk?" (Though I can see the "For" helps one look for a word like "kwa")