This will depend on your starting English proficiency level, practice time with ELSA, frequency of your use (i.e. how often you practice with ELSA). We highly recommend sparing at least 10 minutes a day to practice and review lessons to get the best effect in improving your pronunciation.
Typically, our users show good progress after 4-6 weeks (for those who practice 1-2 hours a day). On the other hand, we find that if you practice 10 minutes a day, 5 days a week, you will see significant improvement after only 3-4 months. This doesn't mean you'll be able to speak as well as Americans right away (language-learning is a long journey after all), but our data shows that you'll make significant progress in your articulation.