How long do I keep my loved one's possessions?

The answer is as individual as you are. Remember, you are in charge. You know what's right for you. 

Just summoning the emotional strength to go through their things can be daunting. I know people whom after 5 - 10 years can't bear to do it. Is that right for me? It doesn't matter. It's not my decision to make and we need to respect others' decisions.
I know of several people, mostly women, who have gone through their child's things, tidied up their rooms and have left them that way. It was their child and their house. They have the right to do what they want.
It's easy for those outside the circle to judge what we do and when we do it. But once inside this dark circle nothing seems to make sense any more. We're guided by our emotions. 
10 years after my daughter's death I can suppress those emotions but they're never really gone. They're not at the forefront of my thinking, normally. 6 weeks after my son's death I'm still in the 7th circle of hell. Emotions guide every move I make; everything I say. I know it will be this way for a while. 
I have kept everything I saved from my daughter's dorm room for 10 years. I have no intention of letting any of it go. There is an exception with my son's things. Because his sons are so young there are things I'm keeping until they're older, like his green beret's and dog tags, for instance. By the time they're old enough to want them I'll probably be senile and won't know the difference. For now I need those things very much.

What I'm saying is that there is no right or wrong answer for the length of time to hold onto possessions. Each of us is different and each circumstance is unique. Keep what you need for as long as you need it.