As long as your definition of massively is flexible, you can knock together a working mmorpg in a few hours if you take advantage of the kit that is available on these forums.
If you have a pot full of money, you can populate your mmorpg with content, some of it AAA, from the Marketplace in short order.
If you want to build everything from the ground up and you keep your world size within reason, you can build a mmorpg from the ground up, creating all of your art assets yourself, in a year or so. (As long as you work full time at it.)
If you are a part time hobbyist, you can do the above in a few years.
It all depends on your OCD level.