I loved my ex-fiance (J). Still do. And not like how I still love most every girl I loved before nor my ex-wife. I was more in love with her (J) than I thought possible. So much so that she (J) still haunts my life. And no she (J) did not die, she (J) just betrayed me like no other ever could. Yet even still, yes I do love her (J).
My ex (C) knew I still loved her (J) and knew she (C) was basicly competing for my love, even though there was no chance in Hell of me ever returning to her (J). My ex (C) did not succeed, nor did she (C) really even try.
My fiance (N) now knows my ex-fiance (J) will always hold a piece of my heart, just as she (N) knows she (N) holds a piece of it, the piece still alive and beating, just for her (N). She (N) knew from the beginning she'd (N) be in a competition and she's (N) succeeded in winning. Winning my heart. Fully.
If the person you love dies, they will always be with you. And if someone comes along who loves you, they will realize that person will always be a part of you and they have to "compete" for your love. Do what they must, not to make you forget, but to make you feel your love again. They may never know the love form you that your original soulmate had, but if they love you they should be happy not only for what they get, but for bringing happiness back into your life.
How long till it's okay for you to move on? If you truly know love, then you never move on. But hopefully you will meet that someone special who helps make you feel alive again.
If you don't know love, or just don't care, then it doesn't matter. Just depends on the person. Hopefully you'll wait till the body is at least buried first, out of respect.
edit- added letters to represent the three ladies in my life and to hopefully clear up any confusion