We don't believe that accents are a problem. There are many examples of effective communicators who have strong accents. Arnold Schwarzenegger, Arianna Huffington, and Penelope Cruz, are three examples.
For most people, the best results are obtained with about 50 hours of training. However, you are free to choose the amount of time you will spend training. If you take a class that is 18 or 24 hours in length and decide that is enough, great. If you would like more training, it will be available.
At the end of training, will your accent be gone? Definitely not. However, you will feel comfortable with your speech and have the tools to be successfully communicating in American English.