The charge service runs automatically every hour. It generates invoices for any charges (recurring invoices) that commence or are scheduled to run during the current charge period.
The charge period is calculated by taking the current date and adding the number of days that were defined in your configured billing rules. See General Settings > Billing Rules > Invoicing – # Days in advance to generate invoices field.
The charge service was designed to make it easy to submit a debit order batch for collection before the date it has to be collected.