One, and only one, if it is directed at me.

That would only happen once with me. You don't get to stay if you say it.

Even one, "I hate you", Not stated playfully is too many. It would be a serious sign of cause for concern. I don't hate anyone and I don't want to hate anyone.

Hearing that directed toward me even once, would prompt me to want a sit down with her to have a sincere heart to heart.

I've been in relationships with women exhibiting toxic patterns and behaviors. So much so that if I ever do find someone normal and mostly sane, I wouldn't know what to do. I grew up in a very toxic environment. I was on the receiving end of a physically abusive relationship with a woman. I didn't do anything or say anything. Apparently my presence was enough for her violence. I thought I 'loved' her. It finally ended when a close friend of mine witnessed her attack me and cut my lip open (also had marks all over my arm) and called 911. My girlfriend at the time went to jail and did two months in jail for domestic abuse.

She used to say she hated me all of the time. She'd balance it with cuteness and "I love you, though".

If playful, let it go. If in anger, sit down like adults and discuss what they meant and what can be done to avoid getting there again. If it happens again, split. You deserve better, and she should realize that.

One, if directed at me.

I just realized, reading your post - that even when things got bad in a relationship, I never said "I hate you" . Nor has anyone ever said it to me.

Guess I never needed that to know the end was near, as it usually shows up in other obvious ways ...

One "I hate you" is plenty for me. Unless you are obviously joking, you just
don't say that.
Telling someone, who you are supposed to love, that you "hate" them is
unacceptable. It's one thing when an immature child says it to a parent.
It's quite another when a grown-ass adult says it to their partner.

Disrespect is disrespect. Period.
That's a deal-breaker for me.

When you tell another person "I love you", whether it's in the context of a romantic relationship, a friendship, or a family member, you have an obligation to treat them
better than anyone else on the fucking planet.
Don't use the words if you aren't going to back it up with word and deed.

Is she 7 years old? I’d say get the hell out now

The one time I heard "I hate you" in a relationship was from a girlfriend when she was cheating on me and wanted to break up but didn't want to come right out and say it. Make of that what you will.

Wow! Those are huge words! I've never said them or had a SO say them. I would think that even once means it's over. I don't think so much vitriol and damage can ever be undone or forgotten, regardless of desire to salvage the relationship.

If such words are used more casually and/or often in a certain pairing (sounds hurtful at least and odd, but no two couples are the same), only those in the relationship can weigh the significance enough to decide.

Depends on a lot of things....intent, circumstances, what happens after that......

Once. (Unless pregnant and pushing a baby out)

I put up with " I hate you" and worse for years and never said it back. But that was not good. I wouldn't accept that today.

Once is enough for irreparable damage. Seems like language of abusive children. I don't suffer that well. Only you can decide how much garbage you can take in a relationship, but rest assured that i hate you is not normal.

My wife and I were saying "I love you" to each other up until the day I wrestled all her mostly new furniture into her apartment. She said, "I love you," then, and I just froze in place, looked at her blankly, and never said it again. I suppose she really was grateful for the thousand bucks in furniture, the truck, and the labor, and that was her way of expressing it. So that's how 27 years ended, and I don't think we ever said "I hate you" to each other, all the way to the bitter end. How much of that was us telling polite lies to each other? Some, to be sure, but even so, I'm going to say that if thousands of "I love yous" ended this way, just one "I hate you" would have been more than sufficient to accelerate this process.

I think River Rick summed it up nicely. Either one saying 'I hate you' is a sure sign it is time to reevaluate you relationship.
Personally I view it as abuse. So it would be if you see me getting smaller it means I'm leaving.

If an "I hate you" comes out and one of the people involved is NOT a 10 year old... Time to call it off.

I hate you is either Very childish, or truly being hurtful with a vengeance! I think One would be cause for serious counseling, or packing.......