Hmmm...that is a tough question. I've got to write a similar scene soon, so I have to think about it too. First thing is that during DH she did cry for Fred when she intially found out. This also depends partially on her relationship with Harry--have they made up yet or are they still apart? Also, though Harry says Ginny is "rarely tearful" that doesn't necessarily anything in this situation. Keeping that in mind, we've got two options:
Main Option 1: Crying
Things to Keep in Mind: She probably wouldn't make a huge deal over it. She's not really the kind to be public and snivel all over the place for sympathy, though depending on her situation, I think she would allow Harry to comfort her. Another thing to keep in mind is her general feelings about the situation and to keep them constant. For example, different people react different ways to grief. If she's just totally given over to it, I suspect she'd just sit there and cry quietly. However, she might be some degree of angry. Perhaps angry at the world in generall, mad at herself for seeming weak, and even mad at Fred for dying. In this case, she'd have a whole lot of other stuff going through her head, and may not let anyone comfort her. Some people get physically sick with grief, and some people lock off, keeping themselves from even speaking ot others, though I don't think necessarily she'd do that. Just something to think about (perhaps she's angry and crying) Yet another thing to think about is guilt. Does she feel guilty over his death? That would also alter the situation.
Main Option 2: Holding Strong
Things to Keep in Mind: Ginny has always been a strong girl, no doubt about it. This is also the kind of more martyr route, giving it all up for others. The thing to keep in mind is not to completely overdo it. Another path you could take with this (a path I've considered) is that she may put her own sort of "blockers" up from reality, forcing herself to keep functioning normally in an attempt to keep both herself and others from hurting. Remember also that she, too, has had a hard year. She's been tortured by the Carrows, no doubt tossed around etc. That could have some impact on her reaction as well. Your third question was if she'd break down after a while of holding strong. I do think that would be her reaction. She'd spend so much energy not crying, forcing herself to take care of others that she'd be sort of denying her own grief. In the end, I think she'll have a massive grief-ridden explosion after so long of holding it in (especially if she feels guilty for Fred's death), though she'd try not to do it publicly.
So, all in all, I think this depends on the kind of Ginny you're trying to depict and the exact situation she's in in terms of guilt, previous hurting (you know, were any of her friends killed? how badly off was she during DH at Hogwarts with the Carrows?), and possibly the situation with Harry. You have to remeber though, pre-war Ginny was spunky, animated, and brave.
Anyway, this is quite the long post and I hope I've helped!