You'll be charged every month at the end of the period depending on how many candidates you have received. You'll be able to access your invoices directly from your job.
You will be able to keep track of the number of candidates you have received and the total spend by heading to "Manage my Job".
Manage Pro Job, here you will find:
The total number of candidates you have received and the cost
Total spend
Current period received candidates, cost and the upcoming charge date charge date
Access to previous monthly charges for that job
Access and download your invoices
Add a budget limit
Pause your job