- Used literally: what length.
- How long is that piece of string?
- I don't know how long the Amazon River is.
- Used literally: what duration.
- How long is the flight?
- How long is it to lunch?
how long (distance)
- Used literally: for what duration.
- How long have you lived in Chicago?
- He won't tell me how long he is going to stay.
how long (length of time)