How long will it take for my old cat to stop hissing at our new kitten?

we got a new kitten about 2 months ago and my old cat, who is 3 years old, is still hissing sometimes at my kitten. she will like breathe heavy (like huff) and then hiss or growl. do i have to worry about them getting into real fights? and now my old cat hisses at me and my family sometimes! like if we try to pick her up, and she never used to do that.