Great friend of mine was applying to a very competitive PA program last year and it was explained to him like this...
After the interview, each candidate is placed into one of three categories:
1. very desirable; acceptance letter or phone call sent within days
2. desirable; acceptance letter sent in weeks
3. not so much; rejection sent in a month
Albeit one programs method is not indicative of the entire system, in my opinion, it is a good yardstick. One factor that might lengthen the delay could be whether or not the school used a rolling admission process, which in my example above was not the case.