I don't know how long is too long, but it's definitely longer than two weeks.
Your boyfriend has experienced one of the most traumatic things a person can experience, the death of a loved one. He's reeling right now and it's going to take him time, obviously, to grieve. Anger is part of grief, which is why he cut you off. He hates the world right now for taking his brother, and you are part of that world, even though he loves you. He's just blinded with fury and sadness.
You seem to understand this, and kudos to you for that, and for being willing to wait for him. That is love in action--not just empty words. I'm sure it's disheartening to be shut out when you want so badly to help him, but there's little you can do until he comes out of shock. Just give him the space he needs, and let him know that you are there for him when he's ready to talk.
How long this will take, I don't know. People grieve and heal in different ways and over varying lengths of time. The death of a sibling is huge, so prepare yourself for a long process. Only you can decide how long you're willing to endure it with him. If you were as happy and in love as you say, then he's worth it.
Just know that this event will change him--for better or for worse--so don't be surprised when both he and your relationship are different. I don't say that to worry you, but simply to temper your expectations of him. Getting over this will take him some time, and he'll need lots of patience and support from you. If the day comes when you get tired of waiting and decide to move on, so be it. That is your right. But it's far too early to even think about that right now.
Good answer, Cary--sympathetic, caring, and well said. Losing a sibling is huge, especially if he's still fairly young and if he was close to his brother, which sounds like the case. My sister and I were very, very close. She died suddenly in a car accident when she was 24 years old, and I was 21. I can't even begin to tell you how devastated I felt when that happened. Grieving and healing take a very long time.
I agree with Daisy...Cary's answer is full of compassion but with realistic expectations for Melissa.
I volunteer with a rescue group and when we trained to be EMT's we learned the five stages of grieving are: denial, anger, bargaining (asking for a second chance to make it right), depression and then acceptance. But the path is not a smooth one and you can stay in some stages longer than others. You can also go back and revisit an earlier stage unexpectedly (which I found out in going through a divorce - which is likened to a death in many ways). It will take time for this guy to heal - I wish her luck in her pain as she waits to see what to do.
Great answer Cary and kudos to Melissa for giving him the space he needs right now. Here's hoping when he's ready he will reach out to you for support.