A couple of guys with a nail gun and some experience should be able to frame the walls in a day or so. I don't know what kind of roof system or wall coverings you'll have so I can't say much more. I did a couple of large scale stables last year. Since horses kick hard I reckon you'll use 2x6 T&G on the insides of the walls and between stalls. That gets to be quite a bit of wood and takes a fair amount of time. Three of us did one twice the size of yours, from the footings to moving in the horses, in a little under three months. It was twenty feet tall and had the T&G I mentioned, a truss roof w/ metal roofing and siding. I think it had about a dozen stalls, a tack room, a feed room and an office upstairs.