"How long is that piece of string?; I don't know how long the Amazon River is."
2.AdjectiveUsed literally: what duration.
"How long are you here for? ~ Until the end of next month.; How long have you already been here (for)? ~ Since last Friday; How long is it to lunch?; How long ago did you stop playing tennis?"
3.AdverbUsed literally: for what duration.
"How long have you lived in Chicago?; He won't tell me how long he is going to stay."