Timing a backstab in Diplomacy
There are lots of good (and bad) times to back stab, I'm not sure there is an out and out correct answer. This is one of the fun aspects of Diplomacy: its variation.

However, early back stabbing is probably almost always best avoided, as you will probably lose credibility with the other players if they think you are unreliable.

Sometimes the best back stabs are against players you have had very long alliances with, when they least expect it.

I also sometimes wait until I have a (usually hidden) alliance with another player, against the same player I am planning to back stab. That way they get hit with a double whammy and you have an extra level of insurance.

Finally, just past the 5th playing hour, as the tiredness and alcohol kicks in, if my experiences are anything to go by, is usually a good time to back stab and lose friends ;)