Closer to 5 years. Substantial work has gone into NL; is it five years of development? Eh, probably not. To be perfectly blunt, a lot of the changes they've implemented for NL are good changes and should have been done to the production servers instead of a new type of server. No, I'm not saying that skill gain should have been converted to quest-based. However, the new skill gain system is sound IF it had been integrated with standard skill gain instead of being two separate tracks. Additionally, the changes to the quest engine are good and would have added a lot to production. Not to mention new art, new encounters, and updated locations.
Now, there are also many changes in NL that were made in a vacuum and, quite frankly, completely misunderstand the player base. I've put a certain user back on ignore, but let me just go ahead and address what I'm sure his white knight response will be: "iTs ThEIr gaMe, YOu dOn'T geT a sAY," and he is correct but ignores the basic rule of running a business (which supposedly he does). If your paying customer base is leaning in one direction for X, you look at what implementing X actually looks like. What are the root pieces of feedback the entire customer base is giving you (e.g., what is the profit and growth opportunity)? This producer goes "I like Y" and implements it with little to no business consideration.
Seasonal servers are quite popular in a wide range of player communities. Unfortunately, in its current configuration, NL didn't implement the right lessons of what makes them successful. What is the purpose of the season? It has to be more than just fun to try out new mechanics. There has to be a goal, a race, or a reward for achieving a particular milestone. The reward needs to be more than just a deco item you can send to production.