How long does it take to download a MMORPG

this highly depends on the MMO/internet speed/client speed. For example an MMO that has been available to the public for a long time will more then likely have numerous patches and updates that will add in some cases several hours to the download.
assuming we are talking about getting the client patcher for an mmo, and then downloading the main content from the client patcher, and not from a dvd/cd then the process could take anywhere from a few short hours to 10 or even more hours. This also depending on your connection.
Games like EQ2, or COH that have been around for 4+ years is a perfect example of this. last time I downloaded COH from scratch it took about 6-8 hours, on a cable connection (12mb/s) start to finish. Not all games are like this, as there clients provide compact updates, that in turn do a one overall update, I believe WoW dose this but can't say for sure, it has been years sense I have played it. non the less these instances are few and far between.
unless you are installing from a disk, and the game is fairly new, it's best to install and start the download from client before you go to sleep so you can wake up with it finished.