Ghostbusters II is not a bad film. However, it was a bit lackluster, and certainly left a sour taste in the mouth of the people involved and the fans, which delayed further sequels indefinitely. Bill Murray is the most important person to be displeased by the film, because he was disappointed at the comedy to such a point where he was reluctant to do another, which resulted in no third film. It has been criticized as too close to the story of the first film, and not having the same quality or 'magic' as the first film. I believe Dan Aykroyd has said they waited too long to do the sequel, and did not capture the momentum and interest that was there after the first film, and that was a problem. The film came out in 1989, which was 5 years after the first. Certainly, that is one of the jokes in the film: they are doing a kids birthday party, and kids could care less because they're old news.
What if Ghostbusters II came about earlier? In traditional Hollywood fashion, one could assume roughly two years after the first film, for a release date of perhaps 1986.