After launching in beta for about 2 months, I added the pricing plans to the app.
I chose the freemium pricing model with limitations on the number of responses and features, especially available integrations. All registered users would get 14-day free trial on the most expensive plan.
The app has 150+ registered users and all of them would get a 90-day trial period (worth $210) instead of 14 days.
Hope it doesn't take more than 2 weeks to post my "First Paying Customer" milestone on IH.