Sweet spot free trail length for subscription apps?

I have a subscription mobile app in the mental health space and I'm experimenting with a free trail period to allow users to benefit from the app for free in hopes that it will motivate them to pay for an annual subscription.

Are there any discoveries others have found when experimenting with free trails for subscription mobile apps? I'm currently experimenting with a 7 day period, but worried that that users will "cool off" on the app in that length of time.