It is a long time since we last met.
=> We haven't seen each other in a long time.  (This is a more natural way to express the idea)

When did you have it?
=> How long have you had it?  (This asks about the duration of possession)

This is the first time I had such a delicious meal.
=> I haven't had such a delicious meal before.  (This emphasizes the novelty of the experience)

I haven't seen him for 8 days.
=> The last time I saw him was 8 days ago.  (This specifies the exact time frame)

I haven't taken a bath since Monday.
=> It is Wednesday, and I haven't taken a bath since Monday.  (This adds context to the statement)