As a native English speaker [USA], I tried "for that" as well, but mainly because I was at a loss as to a more appropriate response (I didn't realize, or forgot, that "bruke" could be used as something other than "use"). "We must use time for that" sounds a little odd, but "We have to use time on it" is even stranger (to me), which is an accepted answer.
A lot of comments here have been wondering if the verb 'bruke' also means 'spend'. It may well do, I'm not sure. I have noticed, however, that in Norwegian verb meanings are often altered when combined with a modal. So, I am wondering if 'må bruke' means 'spend'.
Clarification from a native speaker would be greatly appreciated.