I agree with FREEDOM, We have had intro's as fast as just a couple of minutes and others seem to go on for ever.
Example:
Precious hissed at every body and no one bothered her, now she's fine.
Penny got along with every one
Bailey after 2 years has made peace with every one except Chessie, these two will hiss and fight every chance they get.
We keep squirt bottles all over the house, I am no more than an arms length from one at any time. They all know that I yell once then some one is going to be wet.
I have been known to chase down an offender, none like that. They know that some one is in big trouble.
So it takes time but for the most part things usually settle down.
Good luck and have a little patience