Everyone is different and it can depend on the circumstances surrounding the death. If the bereaved person has had time to say their goodbyes and perhaps has been able to go towards some kind of acceptance, then bereavement tends not to be so long as it might be with other deaths. This is because the healing process that is bereavement is able to begin quite soon after the death of the loved one.
If a loved one has died suddenly due to trauma or violence of some kind, the shock and inability to see them before death can prolong the bereavement process considerably. It is the same, understandably, with parents who loose children - trying to reach a point of acceptance is often impossible for obvious reasons and therefore bereavement is a long process. It is similar with a person who has lost a loved one to suicide. There are so many mixed emotions, so many unanswered questions that bereavement often doesn't follow the healing path that it should.
I would say, in very general terms, that if people are still grieving heavily two years after the loved one's death, then it is likely that some form of bereavement counselling is required. Bereavement counsellers are worth their weight in gold, when helping people to try and find a life to live again after the death of a loved one. But remember that some people might be over the two years and still going through the normal bereavement process, it really just depends.