How long before resident cats stop hissing at new kitten?

[SentientCushion]

So we've decided to keep one of our foster kittens so are trying to integrate them with our two. It's the first time we've ever introduced cats as we usually just keep our fosters separate.

We've been scent swapping and they've lived in the same house for 3 months so I thought it was time they met.
We've been doing slow introductions but we've had a lot of hissing, is this normal or should we go back a stage?

Ive only introduces cats once. I found felliway to be helpful as well.
Think it took a week for ours to settle. I also made a big fuss of first cat. Dont know if that helped or not.

Did yours hiss at the beginning?

I think so. There was lots of chasing at first. Dcat was only 5 months. But after a while they were good friends.

When I last introduced 2 kittens to my 2 (then) adult cats, one of the incumbent cats was absolutely fine and accepted them without issue - the other was extemely grumpy, stopped coming in through the back door, and growled every time he saw the kittens. This lasted three months until one day he just seemed to give up and accepted them. So it can take time - but worth persevering.