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Unless you are behind a router, you can usually determine the life of your IP by calling up a command line (In XP, start, run, cmd then enter), and typing ipconfig /all. I'm not sure what lease time is preset by ntl, although the above would suggest two days.

However, the same IP will be reassigned to you at the end of that lease period if it has not been assigned to another client. Usually in an under utilised region will mean that an IP address will be assigned to the same client for weeks, even months.