The two most time consuming aspects of making a map are creating the elevations and setting the fields in place.
If you're good with external programs like photoshop, terragen, etc elevations can take as little as 15 minutes. If you're not good with them, it could take months to set them by hand.
Setting the fields, again if you're good with the TE it could take a day or two. If you're not it could take a month.
So, as you can see time is relative to expertice.
So far as the order, take a look at the sticky titled MA Terrains in this forum. It should give you some idea of the steps needed.