I think it was several years... I understand the forum has been around since '03, so six years at least, unless there have been other resets I'm unaware of. Most likely there have been.
But even if that lifespan is six years, that's no reason to expect that the current world will last that long. If anything, that Ragnarok party was so awesome that I'm sure not too few wouldn't mind having another one real soon. And since fewer players have giant compounds built up, there's less to loose with such a reset.
In fact, having a regular schedule of universal destruction and resurrection might be a nice idea. I understand that a similar system is used in A Tale in the Desert.