How long does grief last?

That's a trick question.
Grief never ends.
It evolves and transforms you as it does, but it is never over.
Maybe if you are grieving a spouse and you remarry, maybe that is the ultimate transformation of grief. Although I am sure even then you don't stop grieving the spouse you lost. Maybe it becomes more compartmentalized.
But I don't know. I can't speak from experience regarding that because I have not remarried.
And I don't ever intend to.
John and I are soul mates, twin flames, one half of an awesome whole. Any married relationship I could have at this point in my life would be a poor aftermath. It could not ever live up to what John and I had and still have, although it's now changed.
Marriage to John was my home, my refuge, my strength, my comfort, my love. It is only now that John is no longer physically here that I am just beginning to realize how overwhelmingly much we meant to each other and how much we permeated each other's lives. We were JohnandJoy, never apart - not spiritually, not emotionally, and hardly ever physically. We worked together, we played together, we loved together. We were constantly in each other's thoughts - and sometimes even knew each other's thoughts.
We knew our life was good and we appreciated it.
We knew what we had was special.
But as with everything else that is there before you every day, you just assume it will always be there.
Losing John to death leaves me absolutely bereft.
The sadness and loneliness is overwhelming at times, even after two years.
And this sadness of grief is not something that really goes away. Yes, it changes. I don't cry every day. I hardly ever scream any more or pound the furniture in the frustration of grief.
But I ache in my heart. Constantly. Every day.
I yearn for what was.
I see couples going about their day and I feel cheated.
Nothing brings true lasting joy any more. 
Every day I am reminded that John's not here.
John's not here.
John's. Not. Here.
How long does grief last?
Forever.