A classic take on this from the Bard is Much Ado About Nothing (I also recommend this wonderful filmed version, which stays fairly close to the text).
Beatrice and Benedick both swear they will never marry, are not interested in relationships, and are certainly not attracted to each other. They preen and posture and announce and declare, but when their friends and family decide to fix them up, a hint to Benedick that Beatrice likes him but is too shy to say so tips him over immediately.
In more modern romantic comedies, you have two people who profess to be unable to stand one another, but when they are talking with friends, will protest too loudly about some facet of the other person or will admit "Oh, yes, he's certainly handsome/witty/smart/charitable etc., but I could never date him," and the friends correctly read that as "Well, I could if I was talked into it."
If your story is less comedic, then each interaction has to have some element of the wall being chipped away (with one or two reversals so it's not too obvious). In scene 4 she thinks "Wow, we managed to get through a conversation without Dave being an arse." In scene 7 she watches him help an old lady across the street (but doesn't interact with him). In scene 10 he says something which she actually finds funny. Et cetera.
Additionally, your love interest cannot be an actual jerk. He can do things which are annoying to the heroine ("I hate how he dresses like a slob and sits sideways on the furniture!"), but he can't be so antagonistic that the reader doesn't want to see them together (kicking puppies, using homophobic slurs).
As she warms to him, he should also be listening to her complaints and responding to them. So he may not dress in a suit, but maybe he buys new jeans and tucks in his T-shirt.
You need to create a gentle slope of forward progress almost from the beginning, although much of the progress is going to be internal monologue or conversations with characters who are not the love interest, so we see that the heroine's mind is changing even if the love interest doesn't.
The sharp banter can continue throughout, but can be leavened with more genuine compliments, or changed with tone/expression/smiles.
It will feel like it's gone too long if there's no change in her internal monologue about him. If every time she thinks about him or interacts with him her feelings are the same, then he comes off as an annoying stalker.