How much free bandwidth does CDN give me?

It depends on your plan.

Under the Standard Plan, provides free bandwidth quota that resets every month. Beyond that, you are free to purchase additional bandwidth as necessary, without limit. The amount of free bandwidth your site receives is controlled by a Tier System, which depends on the site’s origin server. If you feel that your domain has been assigned an incorrect tier, please follow these steps to reassign it to the correct tier.

Under the Free Plan, all bandwidth is free.