It also depends what they are waiting on. Art assets, animation assets, zone layout, missions, custom bosses, crafting/market infrastructure/UI. Then there is always feature creep from above or within. "Hey product X has this feature, how hard would it be at this point to put it in?"
Biggest problem with software development vs hardware development is the "soft" part. Everyone knows it's just code, no molds, no inventory management, no part or sub-assembly suppliers, no change in the production line. It's easy peasy right?
So is the underlying game client and server code complete and now they are just waiting on game content to be complete? That's always the problem with MMOs, takes months and months to create content and then some dedicated fans blow though it in two weeks or less and demand more.