You can use Starter for as long as you want, but many people complain about how crippled it is and the lack of features.
Read more for useful information.
Aero Glass, meaning you can only use the Windows Basic or other opaque themes. It also means you do not get Taskbar Previews or Aero Peek.
Personalization features for changing desktop backgrounds, window colors, or sound schemes.
The ability to switch between users without having to log off.
64 bit version
Windows Media Center for watching recorded TV or other media.
Remote Media Streaming for streaming your music, videos, and recorded TV from your home computer.
Domain support for business customers.
XP Mode for those that want the ability to run older Windows XP programs on Windows 7.