In English, I might say "How long did the treatment last?", which might ask, how long was it effective for, I might also say "How long did the treatment take?" which would ask how long it took to do the treatment.
How does one differentiate these in Dutch? The hints for duurt seem to suggest both words could apply.
To me, this Dutch sentence only means how long it took to do the treatment. If you want to know how long it is effective for, I would ask "Hoelang werkte de behandeling?".
That might be a more regional thing, so I'll look it up. :)