What's the longest relationship you've been in?

If you mean relationship with a human- that would be my mother.  We've had our ups and downs but so far, so good :).  

I have been with my girlfriend in lives that date back 4,000 years, we were "sisters-in-law" in a nomadic tribe.  We raised the other's children, washed our clothes and cooked our meals together.  We were husband and wife as poor Chinese peasants, but we had a beautiful garden.  We were brothers who sailed a huge ship into Venice during the Renaissance.  We were best friends in the 1700s.  As husband and wife in the 1800s, we owned a dairy farm and made guns for the civil war.   That's probably why I struggled with Atlanta so much- there's a confederate emblem on the State flag.  It would make me shudder and fill with anger- but that effect could be repulsion built up in this life too.

We haven't always been together and it hasn't always been romantic but it's been many lives of all kinds of Love.