As a symbol of its status, the Eclipse was escorted by smaller Super Star Destroyers during the Battle of Pinnacle Base. This also made them high-profile targets by both the Rebel Alliance, and later, the New Republic.
They were estimated to be powerful enough to allow whoever controlled them a real chance at taking power in the galaxy. Their construction took over a decade, with the Emperor's downfall leading to delays and shifts in logistics. The bulk of these two Super-class Star Destroyers was built around each ship's reactor and support systems, making them the most powerful conventional warships in history, and causing the design to attract unwanted attention from criminal third parties.
While conventional armaments were designed for most ships, the two Eclipse-class dreadnoughts had a superlaser as their primary weapon.
It's hard to find a conclusive answer to this question, and obviously, the construction time varies widely depending on the class of ship we're talking about and the political and military situation in the galaxy at the time.
