Many apps in Cloudflare Apps are free and you will not be billed for them, feel free to use them on all your sites free of charge.
All paid apps made available through Cloudflare Apps are billed monthly. You will be charged each month as a part of your normal Cloudflare bill for each zone you have purchased and installed the app on.
If you have an existing Cloudflare billing period you will be automatically charged a discounted pro-rata charge for the first month, aligning your payments for any new app with your normal billing. When you remove an app you are not refunded for the remainder of the final month, but you may reinstall the app at any time during that period to resume using it.