The time it takes to cache (build preview) depends upon the nature of the content.
Individual video clips are cached when the green line above them completes. 
If there are multiple tracks, the sequence is cached when the blue line beneath the preview window turns purple. 
If the Sequence tab that's above the bin (top-left) is clicked, and the sequence previewed, its caching line turns gray.
There won't be a Sequence tab above the bin unless there is more than a single sequence.