How long does it take for resident cats to accept new kittens??

I am trying to introduce two 4-month old kittens to my resident cats: 1-year old mother and 5-month old daughter. I isolated new kittens for a week, and did the towel trick with scent, switch rooms for them, and put up a gate for them to meet face to face. Now it's the 9th day, and the resident cats are still hissing and trying to hit the kittens any chance they get. I tried to put them in the same room for the second time now, and 10 minutes into it the resident cat start hissing and slapping the kittens, and the kittens are very frighten.

Should I keep trying?
Can anyone who had tried this before tell me how long it took?
And how long should I keep them in the same room per day?
If they start fighting, should I "observe" or should I stop them???

Ah.... breaks my heart =(

I have had 2 resident adults meet 2 new kittens...newborn/bottle-fed babies. Fortunately, there was never any hissing or spitting...just some rough play.

I did isolate the kittens like yourself and used a gate for a few days just to protect the kittens. My adult cats were playing rough with them. The towel/scent "trick" seems to help...try using one of the resident's toys letting the kittens play with it & then returning it to the residents. Strings are great for bringing them all together. If you can get a kitten on one end playing and a resident adult on the other end playing...well...sometimes playing with a string...they don't even realize they are playing with one & other until they look up

The hitting may just be a "show" of alpha cat status in your house and not necessarily a display of aggression/anger towards the new kittens.

Although my Buster is a Persian (known for their docile behavior) and is the alpha King in this house, he would practically run over my 2 new kittens in play and would pin them down with power. The pinning down turned into cleaning/licking attacks that were so rough the kittens would cry. My vet. advised me that if Buster really wanted to hurt these kittens, he would have done so already. So...I let them be and all are one big happy family Hope it works out for you...give it some time.

If a cat wants to hurt another cat, you'll know all about it pretty quickly. A bash over the head is just a way of saying "Hey, I was here first and don't you forget it!"

Let them be together supervised until you are satisfied that you can leave them alone together without fearing the worst.

Good luck!

Yeah, I don't think I'd worry too much unless the cats are obviously stressed, or there is bloodshed.

In my household, a week at best. The residents pout and tantrum but eventually realize that they are fun companions. We still get a spat or two, usually between the sisters of all things, but harmony is there.

I got my new kitten (4 month old) 2 weeks ago and in the beginning, my 6 month old would hiss and growl constantly at him. He would bat him too, but never really hurt him. Within 5 days, they were playing and eating together. after 1 week, the younger one would hop into my older cat's favorite chair and the older cat would actually pick up his paw and push the younger cat off. Now at 2 weeks, the older cat has taken to sleeping with me so the younger cat can sleep in the favorite chair.