posted by Franco .
I have a few doubts regarding the use of the present perfect. I really hope you can have a look at them, too.
1) How long ago did you (last?) see him?
2) When did you last see him?
3) How long haven't you seen him? (Is it wrong?)Haven't you seen him for a long time?
4) How long is it since you last saw him?
5) How long have you been in New York.
But How many times have you been to New York?
All are correct, except the first #3 is awkward. The second sentence in # 3 is much better.
You can omit "last" in # 1.