Do you intend to make the game entirely without outside input?
If so, do you already know how to create digital assets? (Sounds, textures, models, etc)
Are you already well versed in Unreal?
If, on the other hand, you must learn all of these things, then you should probably budget 6 months to a year to learn Unreal and somewhat longer to achieve a practical facility in all the other skill sets needed.
In my opinion, once you have put in the time to learn the tools and to learn how to create assets, you could without much difficulty create a working prototype of a single player or small scale multiplayer game within six months of full time effort, a polished prototype within a year and a finished game within one and a half to two years – presuming that you do it all yourself.
The biggest time sink in creating a MMORPG or any game with a large scope is not the coding, but the content creation. The expectation of the average player is that a MMORPG must have a vast depth of content – quests, locations, dungeons, epic story line, and so forth.
However, most importantly, as with all individual endeavors, you will succeed or fail entirely based upon your own dedication and persistence. All that is required is for you to get started, keep working, and finish.