How long do ISPs save IP addresses in the UK?

Does anybody know how long ISPs in the UK (or Virgin Media) save the data they collect like IP addresses and websites visited? I can't find any information regarding this on the Virgin website.

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia��).