As a fellow English rider, and male, I will tell you it does take quite a bit of time. I think it took about a month before I could post the trot. A lot of good advice out there on this post, but the most important is to just stick with it and practice, practice, practice. I picked up riding as an adult and it takes a tremendous amount of patience and challenges you both mentally and physically. As others have posted, it depends not only on you but also the horse.
Private lessons and sticking with the same horse to begin with will help. Lunge lessons help tremendously, especially with learning a new gait, like rising trot. You'll have the opportunity to really focus on getting it right without having to worry as much about steering and all of the other headaches.