Support for Win XP SP3 will last until April 2014. You can continue to use beyond that date if you wish, Microsoft will simply stop providing updates and support. A friend of mine stated that the Gov't agencies like the Military Intelligence Agency are still using Windows 98 as well as some normal citizens.
Are there any particular reason why you want to upgrade to Windows 7? DirectX 11 for games, you just want something shiny, etc....
Will Windows 7 "live" as long as Win XP (13 years life cycle)? Unknown, but unlikely.