Some people might get over it fairly quickly, but in most cases it takes a long time. And that time could differ depending on the person, and depending on who they lost. For instance, if my girlfriend died, I honestly don't know if I'd ever fully get over the grieving stage, however she's an exception to me. My condolences to you, or the person you know who's grieving. But don't ever listen to people who tell you that you should just get over it, cause that's a load of crap.
Grieving will be different for everyone. Some people will loose a child and not even cry and people will say the mother didn't care, or she caused the death some how, when in reality the mother is still in shock. Their is no acceptable time line or benchmarks for anyone at anytime in the grieving process. Many people will never "get over it". I'd say time will help and to occupy yourself with good productive things to keep your mind clear. Most funeral homes offer free help for people who are grieving like booklets and videos. I worked as a funeral director and embalmer for a few years and saw grieving that I wouldn't even believe myself. We had one lady in her 70's loose her husband, 3 children and 5-Grandkids in the matter of one year. Grieving can be more severe in the death of a child, or a child who looses his mother. Lives can be shattered, but help does exist. Time alone seems to help more than anything alone.