Dissection. Aortic dissection is a life threatening condition but an occassional patient will have evidence for a past dissection without having been hospitalized. Extrapolating from history this can sometimes be a long time. We don't really know how often this happens but believe it to be rare.
Years. Depending on where the dissection occurs and if it is expanding. As long as the dissection is in the descending aorta and isn't causing any symptoms - just good blood pressure control is all that may be needed.