When will memories bring comfort?

It's been six since our son died.. I think it took several months for reality to sink in. In fact, I'm starting to think our grief has two parts. I am still grieving the last year of our son's life. I keep reliving all those difficult months of caring for him, and how it was watching him endure all the losses his brain cancer caused: his independence, his job, driving, and all the activities he liked to do. And then the anguish of watching his physical decline. They say that memories bring comfort, but for me, they just cause pain and sadness.
Maybe some day the grief will center more on his absence and then even move on to where good memories will bring comfort. Maybe then I will feel a healing. How long does that take?