When you install Google Desktop for Linux, it immediately begins indexing the email, files, and webpages already stored on your computer. By default, the indexing process occurs when your computer is idle and may take up to several hours or even overnight (possibly longer if you shut down your computer during the initial indexing).
Once this initial index is complete, Google Desktop automatically keeps up with any changes you make. When you open, save, view, or move something on your computer, Google Desktop updates your index. This item becomes searchable within seconds, so you don't need to manually update your index.
(Note: Only kernel versions 2.6.13 and up support immediate real-time indexing. For Kernels lower than 2.6.13, Google Desktop will refresh the index periodically to keep you up to date).