As you probably know, Spotlight works by indexing your hard drive. When you first install OS X, and at other times (installing a new drive, changing the Spotlight Privacy settings), Spotlight creates a new index for your system. This process can take a lot of time—especially if you’ve got a larger hard drive loaded with tons of smaller files.
OS X tries to help you out by telling you how much time is left in the indexing operation—just click the Spotlight icon in the top right corner of the menu bar, and you’ll see both a progress indicator and a written estimate of the time remaining (“About two hours remaining”). The problem with the written estimate is that it can vary quite a bit, changing by minutes or sometimes hours for no apparent reason. The progress bar is better in that regard, as it gives you a visual on the percentage of completion.
There’s actually one more visual indicator you can see—the exact percentage of your drives that have been indexed, as seen in the image to the lower right.