A. What's going on?
In BrE this normally means, "what is happening/has happened?" It is often asked when there are signs of unusual activity.
- B. It's been a long time since I have visited here.
It's acceptable, though with 'here', these sound more natural to me:
It's a long time since I have been here
It's a long time since I was here.
I haven't been here for a long time.