It depends on the scale of your dress. If it's a simple dress and you have a lot of time on your hands, sure! But the larger the scale is, the longer the time will take.
If you have previous experience with aerial tricks of any kind, then it shouldn't take long. However, if you're new to aerials or you don't have a strong core, it...
You could start training him now by getting him accustomed to having weight on his back, basically turning him into a pack-steer. But, he won't be ready to actually ride...