How long is too long? It really isnt ever over. Okay so the relationship has died; thats over, but the memories etc aren't. They never will be.
And when is the right time to let go? Everyone is so different. Two weeks, two months, two years? How ever long it takes, how ever much time it takes to move on, is completely up to you. No ones imput is needed and is completely unnecessary.
And whether you decide to remain friends is too your own decision. For some it can be too hard to remain friends with someone who meant alot to you. To see them move on and take thoses first steps into a new relationship can be hard. And in that case, your "Over" is alot closer to "over" then those who remain friends. But remaining friends is also a way to still have the person. But do you still have feelings? If you still have feelings and are both still friends, it sort of gives you false hope of reconcile. So double heart break/ache.
But when you stop thinking about that person, stop thinking about a reconcile or reunion, its great!
You think for yourself and yourself only, even though you have departed a relationship from that person, you still feel like you have to think about them in what you do daily. It is the best feeling when over is as "Over" as it can be.
Life begins again!