How long would an normal echocardiogram be efective against aortic dissection. In other words would a normal echo rule out future dissection?

No, but... Sadly, no. An echo can only see the world as it is at that exact moment in time. Dissection can always occur later. That said, thoracic dissections are rarely both serious and asymptomatic, so if one started, odds are good that you'd know in a painful, let's go visit the er sort of way.