I used the prong collar as well with my Betty until she got heeling down. I didn't use it as a correction tool, as in me correcting her with leash pops, but as a self-correcting tool, where she corrected herself. I would say that it took about 4 months, and she was walking nicely, and heeling well in Rally class. So after this, I started phasing out the prong collar by putting on both the regular collar and the prong, and having the leash on the regular collar. Then eventually I wasn't having to put on the prong any more. But I did have a couple instances where she would start pulling again, and I put the prong back on for a session, then back to the regular collar. After a couple times, I no longer needed to use the prong, and to this day, have not even taken it out.
There's a few people on here that don't like prongs, and they have their own opinions. But from my view, it is a very useful tool if used properly. And once you get your dog to the point to where she's walking nicely, you can phase the prong out and go back to the standard collar.
As for injuring the dog, the prong does not poke through the dogs skin if properly used. And if you decided to use my technique where the dog corrects itself, when the dog starts to go ahead and pull, it will feel uncomfortable and the dog will slow back down. Eventually the dog won't pull anymore. And if you've done some clicker training, you can always click and treat everytime the dog slows back down, and she will probably learn a lot faster. I wasn't clicker training at the time I worked with Betty, and wish I had been, would have been much faster. Betty and clicker training has been awsome.