My answer has always been that it takes as long as it takes. Every animal and every human work and learn at different speeds. I have had some except everything that I did in really quick time frames, then I have had some that take what seems like forever to accept any new things. I never put a time frame on it and this way I don't add pressure to the horse or myself. It just happens at the speed in which the horse can learn and also based on how much the horse trusts you.
Added: I like to spend a ton of time on ground work. I probably spend more than most, but since this is "the foundation on which the house stands", it is important that it is very solid. I typically spend several months on the ground work on the large riding horses before getting into the saddle the first time, and I firmly believe that the minis require the same steps to be totally understood before getting in a cart. But, where I may hurry up steps in a riding horse, I won't in a driving as I don't have anything but the reins and my voice to control a driving horse. To me, it is extremely important that the horse firmly grasps the concepts of everything if it will be driving for safety reasons.
As a comparison, I had a mare take 2 months before driving in her first show. I had another mare of identical age take twice that long and still not be ready to go to a show. It is totally dependant on the horse, the person training, and the partnership between the two.