The answer to your question would appear to be 'very soon'!
From the Virgin Media website:
"As a Virgin Media customer, you pay for your services a month in advance. For example, you pay in April for May’s services.
There are only 2 bits of your bill that work differently:
* TV extras, like Movies On Demand or sports events, show up on your bill the following month.
* Phone call charges will appear on your bill the month after the calls were made.
All service charges are listed separately on your bill, except premium TV channels and Virgin Phone call features, which are shown as a combined amount.
All service charges on your bill include VAT, except phone call charges, which are shown as a total amount excluding VAT."
Chris