Based on your description of the app I would suggest that you need some users before you can even plan for charging them. Once you have some initial users they will do the word of mouth marketing and help you spread the word faster. So take in account the money saved in marketing.

Charging or not charging also really depends how much cash your company has and how much longer your startup can survive without making any money from it. Whatever is the time estimate, deduct 3 to 6 months from it and start charging. Timing this is the essential part of the strategy.