In short, Iwtk, you can omit for
and still have a grammatical question.
What Seiichi is saying is that we can choose to use for
in front of the time phrase how long
, or we can omit it:It depends on for how long you want the car.
It depends on how long you want the car.
If we choose to keep for
, we normally postpose it (place it after everything) and sayIt depends on how long want the car for.