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.
Windows 8 is set to be released within a few years. But does that mean Windows 7 wont last a while? Nah. Its hard to say unless Windows 8 makes a revolutionary step such as a new files system (WINFS) that performs better. Or it might just be a simple performance upgrade.
If anything though Windows 7 alone has benefits. Better multicore utilization and better memory management are some of the benefits over XP.