Excessive usage is handled differently between Managed and Pay-As-You-Go subscriptions as detailed below.

For Managed daily usage

Users who exceed their plan's chosen daily usage limit will be automatically upgraded to the next daily limit tier to better accommodate their usage (IE, from a 3-hour daily limit to a 5-hour daily limit).

For Pay-As-You-Go prepaid usage

If your usage exceeds your prepaid amount, your account will be automatically billed according to the plan you have chosen. 

Further information

If you wish to read more about these policies, please review the links below: